[AST] Break dependency cycle between swiftAST and swiftClangImporter

NormalProtocolConformance::isRetroactive() introduces dependency on swiftClangImporter by calling ClangModuleUnit::getAdapterModule().
Do some refactoring to break the cycle.

Author: akyrtzi

include/swift/AST/Module.h

@@ -1315,6 +1315,9 @@ class LoadedFile : public FileUnit {
     return nullptr;
   }
 
+  /// Returns the Swift module that overlays a Clang module.
+  virtual ModuleDecl *getAdapterModule() const { return nullptr; }
+
   virtual bool isSystemModule() const { return false; }
 
   /// Retrieve the set of generic signatures stored within this module.

include/swift/ClangImporter/ClangModule.h

@@ -54,7 +54,7 @@ class ClangModuleUnit final : public LoadedFile {
   bool isTopLevel() const;
 
   /// Returns the Swift module that overlays this Clang module.
-  ModuleDecl *getAdapterModule() const;
+  ModuleDecl *getAdapterModule() const override;
 
   /// Retrieve the "exported" name of the module, which is usually the module
   /// name, but might be the name of the public module through which this

lib/AST/ProtocolConformance.cpp

@@ -24,7 +24,6 @@
 #include "swift/AST/Types.h"
 #include "swift/AST/TypeWalker.h"
 #include "swift/Basic/Statistic.h"
-#include "swift/ClangImporter/ClangModule.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/TinyPtrVector.h"
@@ -437,10 +436,10 @@ bool NormalProtocolConformance::isRetroactive() const {
 
     // Consider the overlay module to be the "home" of a nominal type
     // defined in a Clang module.
-    if (auto nominalClangModule =
-          dyn_cast<ClangModuleUnit>(nominal->getModuleScopeContext())) {
+    if (auto nominalLoadedModule =
+          dyn_cast<LoadedFile>(nominal->getModuleScopeContext())) {
       if (auto clangLoader = nominal->getASTContext().getClangModuleLoader()) {
-        if (auto overlayModule = nominalClangModule->getAdapterModule())
+        if (auto overlayModule = nominalLoadedModule->getAdapterModule())
           nominalModule = overlayModule;
       }
     }
@@ -454,7 +453,9 @@ bool NormalProtocolConformance::isRetroactive() const {
 }
 
 bool NormalProtocolConformance::isSynthesizedNonUnique() const {
-  return isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext());
+  if (auto *file = dyn_cast<FileUnit>(getDeclContext()->getModuleScopeContext()))
+    return file->getKind() == FileUnitKind::ClangModule;
+  return false;
 }
 
 bool NormalProtocolConformance::isResilient() const {