Change getRedeclContext() to support enumerations as another kind of transparent context in C.

This change fixes PR15071 and ensures that enumerators redefined in a struct cannot conflict with enumerators defined outside of the struct.

llvm-svn: 345073
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index f5de359..f65d306 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1724,8 +1724,18 @@
 
 DeclContext *DeclContext::getRedeclContext() {
   DeclContext *Ctx = this;
-  // Skip through transparent contexts.
-  while (Ctx->isTransparentContext())
+
+  // In C, a record type is the redeclaration context for its fields only. If
+  // we arrive at a record context after skipping anything else, we should skip
+  // the record as well. Currently, this means skipping enumerations because
+  // they're the only transparent context that can exist within a struct or
+  // union.
+  bool SkipRecords = getDeclKind() == Decl::Kind::Enum &&
+                     !getParentASTContext().getLangOpts().CPlusPlus;
+
+  // Skip through contexts to get to the redeclaration context. Transparent
+  // contexts are always skipped.
+  while ((SkipRecords && Ctx->isRecord()) || Ctx->isTransparentContext())
     Ctx = Ctx->getParent();
   return Ctx;
 }