Consistently initialize clang CodeGenOptions.DebugTypeExtRefs

on all code paths. This flag is part of the Clang module hash. Setting it on all
paths makes it easier to reason about the permutations of Clang modules that are
being built on behalf of Swift.

(cherry picked from commit f0b7065)

Author: adrian-prantl

lib/IRGen/IRGenModule.cpp

@@ -103,6 +103,7 @@ static clang::CodeGenerator *createClangCodeGenerator(ASTContext &Context,
   auto &CGO = Importer->getClangCodeGenOpts();
   CGO.OptimizationLevel = Opts.shouldOptimize() ? 3 : 0;
 
+  CGO.DebugTypeExtRefs = !Opts.DisableClangModuleSkeletonCUs;
   CGO.DiscardValueNames = !Opts.shouldProvideValueNames();
   switch (Opts.DebugInfoLevel) {
   case IRGenDebugInfoLevel::None:
@@ -113,7 +114,6 @@ static clang::CodeGenerator *createClangCodeGenerator(ASTContext &Context,
     break;
   case IRGenDebugInfoLevel::ASTTypes:
   case IRGenDebugInfoLevel::DwarfTypes:
-    CGO.DebugTypeExtRefs = !Opts.DisableClangModuleSkeletonCUs;
     CGO.setDebugInfo(clang::codegenoptions::DebugInfoKind::FullDebugInfo);
     break;
   }