Look for keywords in inactive #if regions when checking for variable uses

Fixes issue #79555

Author: DougGregor

lib/ASTGen/Sources/ASTGen/CompilerBuildConfiguration.swift

@@ -359,11 +359,15 @@ private enum InactiveCodeChecker {
       // match.
       switch self {
       case .name(let name):
-        guard let identifier = token.identifier, identifier.name == name else {
-          continue
+        if let identifier = token.identifier, identifier.name == name {
+          break
         }
 
-        break
+        if case .keyword = token.tokenKind, token.text == name {
+          break
+        }
+
+        continue
 
       case .tryOrThrow:
         guard let keywordKind = token.keywordKind,

test/decl/var/usage.swift

@@ -571,3 +571,16 @@ func testEnumeratedForLoop(a: [Int]) {
     let _ = c
   }
 }
+
+// https://github.com/swiftlang/swift/issues/79555
+final class A {
+  var x: () -> Void {
+    { [weak self] in // Used to warn: variable 'self' was written to, but never read
+      #if NOT_PROCESSED
+      self?.f()
+      #endif
+    }
+  }
+
+ func f() {}
+}