[PCH][OutputFileMap] GeneratePCH output should use single output entry

The build system has been emitting the output path of generating PCH job
in the module level entry in the output file map, while swift-driver is
expecting the entry in an entry that accosiated with imported bridging
header.

This patch unifies the behavior by picking the implementation of the
swift-build, where it is a module level entry. This works better for
future when bridging header can be chained, thus creating PCH job even
there isn't a imported objc header explicitly provided.

rdar://146395316

Author: cachemeifyoucan

Sources/SwiftDriver/Driver/Driver.swift

@@ -3133,8 +3133,7 @@ extension Driver {
   func computePrecompiledBridgingHeaderDir(
     _ parsedOptions: inout ParsedOptions,
     compilerMode: CompilerMode) throws -> VirtualPath? {
-    if let input = originalObjCHeaderFile,
-       let outputPath = try? outputFileMap?.existingOutput(inputFile: input, outputType: .pch) {
+    if let outputPath = try? outputFileMap?.existingOutputForSingleInput(outputType: .pch) {
       return VirtualPath.lookup(outputPath).parentDirectory
     }
     if let outputDir = parsedOptions.getLastArgument(.pchOutputDir)?.asSingle {
@@ -3154,7 +3153,7 @@ extension Driver {
         return nil
     }
 
-    if let outputPath = try? outputFileMap?.existingOutput(inputFile: input, outputType: .pch) {
+    if let outputPath = try? outputFileMap?.existingOutputForSingleInput(outputType: .pch) {
       return outputPath
     }
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -1433,6 +1433,38 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+  func testEmitPCHWithOutputFileMap() throws {
+    try withTemporaryDirectory { path in
+      let outputFileMap = path.appending(component: "outputFileMap.json")
+      try localFileSystem.writeFileContents(outputFileMap, bytes: """
+        {
+          "": {
+            "pch": "/build/Foo-bridging-header.pch"
+          }
+        }
+        """
+      )
+      var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Foo", "-emit-module",
+                                      "-serialize-diagnostics", "-experimental-emit-module-separately",
+                                      "-import-objc-header", "bridging.h", "-enable-bridging-pch",
+                                      "-output-file-map", outputFileMap.description])
+      let plannedJobs = try driver.planBuild().removingAutolinkExtractJobs()
+      XCTAssertTrue(driver.diagnosticEngine.diagnostics.isEmpty)
+
+      // Test the output path is correct for GeneratePCH job.
+      XCTAssertEqual(plannedJobs.count, 4)
+      XCTAssertEqual(plannedJobs[0].kind, .generatePCH)
+      try XCTAssertJobInvocationMatches(plannedJobs[0], .flag("-o"), .path(.absolute(.init(validating: "/build/Foo-bridging-header.pch"))))
+
+      // Plan a build with no bridging header and make sure no diagnostics is emitted (pch in output file map is still accepted)
+      driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Foo", "-emit-module",
+                                  "-serialize-diagnostics", "-experimental-emit-module-separately",
+                                  "-output-file-map", outputFileMap.description])
+      let _ = try driver.planBuild()
+      XCTAssertTrue(driver.diagnosticEngine.diagnostics.isEmpty)
+    }
+  }
+
   func testReferenceDependencies() throws {
     var driver = try Driver(args: ["swiftc", "foo.swift", "-incremental"])
     let plannedJobs = try driver.planBuild()