Replace Sema's map of locally-scoped extern "C" declarations with a DeclContext
of extern "C" declarations. This is simpler and vastly more efficient for
modules builds (we no longer need to load *all* extern "C" declarations to
determine if we have a redeclaration).

No functionality change intended.

llvm-svn: 231538
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index fb72a9a..f79d137 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -34,6 +34,7 @@
 void CodeGenFunction::EmitDecl(const Decl &D) {
   switch (D.getKind()) {
   case Decl::TranslationUnit:
+  case Decl::ExternCContext:
   case Decl::Namespace:
   case Decl::UnresolvedUsingTypename:
   case Decl::ClassTemplateSpecialization: