[AST] Hack SourceFile::lookupLocalType() to look through local types.

This is only currently exercised by swift-remoteast-test, so do the
minimum to ensure that we’re getting cached mangled names, but don’t
fret over the linear-time search.

lib/AST/Module.cpp

@@ -32,6 +32,7 @@
 #include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/PrintOptions.h"
 #include "swift/AST/ProtocolConformance.h"
+#include "swift/AST/TypeCheckRequests.h"
 #include "swift/Basic/Compiler.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
@@ -565,6 +566,22 @@ void SourceFile::getLocalTypeDecls(SmallVectorImpl<TypeDecl*> &Results) const {
   Results.append(LocalTypeDecls.begin(), LocalTypeDecls.end());
 }
 
+TypeDecl *SourceFile::lookupLocalType(llvm::StringRef mangledName) const {
+  ASTContext &ctx = getASTContext();
+  for (auto typeDecl : LocalTypeDecls) {
+    auto typeMangledName = evaluateOrDefault(ctx.evaluator,
+                                             USRGenerationRequest { typeDecl },
+                                             std::string());
+    if (typeMangledName.find("s:") == 0)
+      typeMangledName = typeMangledName.substr(2);
+
+    if (mangledName == typeMangledName)
+      return typeDecl;
+  }
+
+  return nullptr;
+}
+
 void ModuleDecl::getDisplayDecls(SmallVectorImpl<Decl*> &Results) const {
   // FIXME: Should this do extra access control filtering?
   FORWARD(getDisplayDecls, (Results));