[IRGen] Fetch the correct IGM for synthesized files

rdar://128870792

Synthesized files are treated as not having a parent source file, but that can cause issues for macro code. The declarations will have the source file of their use site declared as parent source file, which means they should be emitted into the same IGM. If we don't use the correct IGM, we can get inconsistencies when referencing the generated code, which will cause linking issues.

Author: drexin

lib/IRGen/IRGen.cpp

@@ -1440,7 +1440,7 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
       }
 
       if (auto *synthSFU = File->getSynthesizedFile()) {
-        CurrentIGMPtr IGM = irgen.getGenModule(synthSFU);
+        CurrentIGMPtr IGM = irgen.getGenModule(&synthSFU->getFileUnit());
         IGM->emitSynthesizedFileUnit(*synthSFU);
       }
     } else {

test/Macros/extension_emission.swift

@@ -0,0 +1,61 @@
+// REQUIRES: swift_swift_parser
+
+// This test ensures that code generated by extension macros gets emitted into the correct LLVM module
+// rdar://128870792
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -parse-as-library -module-name=MacroDefinition %t/macro.swift -g -no-toolchain-stdlib-rpath
+
+// RUN: %target-swift-frontend -num-threads 1 -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -enable-library-evolution %t/b.swift %t/a.swift -emit-ir -o %t/b.ll -o %t/a.ll
+
+// RUN: %FileCheck -check-prefix=CHECK-A %s < %t/a.ll
+
+// RUN: %FileCheck -check-prefix=CHECK-B %s < %t/b.ll
+
+//--- macro.swift
+
+import SwiftSyntax
+import SwiftSyntaxBuilder
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
+
+public struct SomeExtensionMacro: ExtensionMacro {
+  public static func expansion(of node: AttributeSyntax, attachedTo declaration: some DeclGroupSyntax, providingExtensionsOf type: some TypeSyntaxProtocol, conformingTo protocols: [TypeSyntax], in context: some MacroExpansionContext) throws -> [ExtensionDeclSyntax] {
+    let decl: DeclSyntax =
+    """
+    extension \(type.trimmed) {
+      struct Storage {
+        let x: Int
+      }
+
+      func alsoUseStorage(_ x: Storage?) {
+        print(type(of: x))
+      }
+    }
+    """
+    guard let extensionDecl = decl.as(ExtensionDeclSyntax.self) else {
+      return []
+    }
+
+    return [extensionDecl]
+  }
+}
+
+//--- a.swift
+
+@attached(
+  extension,
+  names: named(Storage), named(alsoUseStorage)
+)
+macro someExtension() = #externalMacro(module: "MacroDefinition", type: "SomeExtensionMacro")
+
+// CHECK-A: @"$s1b1SV7StorageVMn" = {{.*}}constant
+@someExtension
+struct S {}
+
+//--- b.swift
+
+// This file needs no content, it just needs to exist
+
+// CHECK-B-NOT: @"$s1b1SV7StorageVMn"