Fix LTO support for WebAssembly targets (#1710)

It seems .bc input file handling and `-flto` clang linker driver option were forgotten to be ported.

Author: kateinoigakukun

Sources/SwiftDriver/Jobs/WebAssemblyToolchain+LinkerSupport.swift

@@ -106,6 +106,8 @@ extension WebAssemblyToolchain {
           return .responseFilePath(input.file)
         } else if input.type == .object {
           return .path(input.file)
+        } else if lto != nil && input.type == .llvmBitcode {
+          return .path(input.file)
         } else {
           return nil
         }
@@ -162,6 +164,15 @@ extension WebAssemblyToolchain {
         throw Error.profilingUnsupportedForTarget(targetTriple.triple)
       }
 
+      if let lto = lto {
+        switch lto {
+        case .llvmFull:
+          commandLine.appendFlag("-flto=full")
+        case .llvmThin:
+          commandLine.appendFlag("-flto=thin")
+        }
+      }
+
       // Run clang++ in verbose mode if "-v" is set
       try commandLine.appendLast(.v, from: &parsedOptions)
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2172,6 +2172,27 @@ final class SwiftDriverTests: XCTestCase {
       let linkJob3 = try plannedJobs3.findJob(.link)
       XCTAssertTrue(linkJob3.tool.name.contains("clang"))
       XCTAssertTrue(linkJob3.commandLine.contains(.flag("-flto=full")))
+
+      try withTemporaryDirectory { path in
+        try localFileSystem.writeFileContents(path.appending(components: "wasi", "static-executable-args.lnk")) {
+          $0.send("garbage")
+        }
+        var driver4 = try Driver(args: commonArgs + [
+          "-emit-executable", "-target", "wasm32-unknown-wasi", "-lto=llvm-thin", "baz.bc",
+          "-resource-dir", path.pathString
+        ], env: env)
+        let plannedJobs4 = try driver4.planBuild()
+        XCTAssertFalse(plannedJobs4.containsJob(.autolinkExtract))
+        let linkJob4 = try plannedJobs4.findJob(.link)
+        XCTAssertTrue(linkJob4.tool.name.contains("clang"))
+        XCTAssertTrue(linkJob4.commandLine.contains(.flag("-flto=thin")))
+        for linkBcInput in ["foo", "bar", "baz.bc"] {
+          XCTAssertTrue(
+            linkJob4.inputs.contains { $0.file.basename.hasPrefix(linkBcInput) && $0.type == .llvmBitcode },
+            "Missing input \(linkBcInput)"
+          )
+        }
+      }
     }
 
     do {