SwiftPM complains about @main in non main.swift file when integrated driver is not enabled (#3546) (#3575)

One of several different code paths in BuildPlan.swift are taken depending on whether the integrated Swift driver is enabled, whether explicit modules are enabled, etc.

In the non-integrated-driver case, the code path for adding a `-parse-as-library` when necessary wasn't taken.  This moves that code to a more central place, and extends a unit test to test both single and multiple file Swift targets, which seems to trigger the difference in code paths.

rdar://78080245
(cherry picked from commit e84fd1c)

Author: abertelrud

Fixtures/Miscellaneous/AtMainSupport/Package.swift

@@ -1,14 +1,16 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.5
 import PackageDescription
 
 let package = Package(
     name: "AtMainSupport",
     products: [
-        .executable(name: "ClangExec", targets: ["ClangExec"]),
-        .executable(name: "SwiftExec", targets: ["SwiftExec"]),
+        .executable(name: "ClangExecSingleFile", targets: ["ClangExecSingleFile"]),
+        .executable(name: "SwiftExecSingleFile", targets: ["SwiftExecSingleFile"]),
+        .executable(name: "SwiftExecMultiFile", targets: ["SwiftExecMultiFile"]),
     ],
     targets: [
-        .executableTarget(name: "ClangExec"),
-        .executableTarget(name: "SwiftExec"),
+        .executableTarget(name: "ClangExecSingleFile"),
+        .executableTarget(name: "SwiftExecSingleFile"),
+        .executableTarget(name: "SwiftExecMultiFile"),
     ]
-)
+)

Fixtures/Miscellaneous/AtMainSupport/Sources/ClangExec/NotMain.c renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/ClangExecSingleFile/NotMain.c

@@ -0,0 +1,6 @@
+@main 
+struct MyProgram {
+    static func main() {
+        print("Hello, Swift.")
+    }
+}

Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExec/NotMain.swift renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExecMultiFile/NotMain.swift

@@ -757,6 +757,11 @@ public final class SwiftTargetBuildDescription {
                 }
             }
         }
+        
+        // If the target needs to be parsed without any special semantics involving "main.swift", do so now.
+        if self.needsToBeParsedAsLibrary {
+            args += ["-parse-as-library"]
+        }
 
         // Only add the build path to the framework search path if there are binary frameworks to link against.
         if !libraryBinaryPaths.isEmpty {
@@ -809,10 +814,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         if buildParameters.useWholeModuleOptimization {
             result.append("-whole-module-optimization")
             result.append("-num-threads")
@@ -849,10 +850,6 @@ public final class SwiftTargetBuildDescription {
         result.append("-experimental-skip-non-inlinable-function-bodies")
         result.append("-force-single-frontend-invocation")
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         // FIXME: Handle WMO
 
         for source in target.sources.paths {
@@ -896,9 +893,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
         // FIXME: Handle WMO
 
         result.append("-c")

Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExec/OtherFile.swift renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExecMultiFile/OtherFile.swift

@@ -176,15 +176,22 @@ final class BuildToolTests: XCTestCase {
         fixture(name: "Miscellaneous/AtMainSupport") { path in
             let fullPath = resolveSymlinks(path)
             do {
-                let result = try build(["--product", "ClangExec"], packagePath: fullPath)
-                XCTAssert(result.binContents.contains("ClangExec"))
+                let result = try build(["--product", "ClangExecSingleFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("ClangExecSingleFile"))
             } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
                 XCTFail(stdout + "\n" + stderr)
             }
 
             do {
-                let result = try build(["--product", "SwiftExec"], packagePath: fullPath)
-                XCTAssert(result.binContents.contains("SwiftExec"))
+                let result = try build(["--product", "SwiftExecSingleFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("SwiftExecSingleFile"))
+            } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
+                XCTFail(stdout + "\n" + stderr)
+            }
+
+            do {
+                let result = try build(["--product", "SwiftExecMultiFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("SwiftExecMultiFile"))
             } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
                 XCTFail(stdout + "\n" + stderr)
             }