Add API to get the "outermost" parent source file.

Author: DougGregor

include/swift/AST/DeclContext.h

@@ -519,6 +519,12 @@ class alignas(1 << DeclContextAlignInBits) DeclContext
   LLVM_READONLY
   SourceFile *getParentSourceFile() const;
 
+  /// Returns the "outermost" source file that contains this context,
+  /// looking through any source files for generated code, such as
+  /// macro expansions.
+  LLVM_READONLY
+  SourceFile *getOutermostParentSourceFile() const;
+
   /// Determine whether this declaration context is generic, meaning that it or
   /// any of its parents have generic parameters.
   bool isGenericContext() const;

lib/AST/DeclContext.cpp

@@ -323,6 +323,18 @@ SourceFile *DeclContext::getParentSourceFile() const {
   return fallbackSF;
 }
 
+SourceFile *DeclContext::getOutermostParentSourceFile() const {
+  auto sf = getParentSourceFile();
+  if (!sf)
+    return nullptr;
+
+  // Find the originating source file.
+  while (auto enclosingSF = sf->getEnclosingSourceFile())
+    sf = enclosingSF;
+
+  return sf;
+}
+
 DeclContext *DeclContext::getModuleScopeContext() const {
   auto DC = const_cast<DeclContext*>(this);
 

lib/IRGen/IRGenModule.cpp

@@ -1919,10 +1919,11 @@ IRGenModule *IRGenerator::getGenModule(DeclContext *ctxt) {
   if (GenModules.size() == 1 || !ctxt) {
     return getPrimaryIGM();
   }
-  SourceFile *SF = ctxt->getParentSourceFile();
+  SourceFile *SF = ctxt->getOutermostParentSourceFile();
   if (!SF) {
     return getPrimaryIGM();
   }
+
   IRGenModule *IGM = GenModules[SF];
   assert(IGM);
   return IGM;