merge hasCLanguageLinkage and isExternC. Keep the shorter name.

I added hasCLanguageLinkage while fixing some language linkage bugs some
time ago so that I wouldn't have to check all users of isExternC. It turned
out to be a much longer detour than expected, but this patch finally
merges the two again. The isExternC function now implements just the
standard notion of having C language linkage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175119 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 9c7fb6b..d2b4697 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -4860,7 +4860,7 @@
 template<typename T>
 static bool mayConflictWithNonVisibleExternC(const T *ND) {
   VarDecl::StorageClass SC = ND->getStorageClass();
-  if (ND->hasCLanguageLinkage() && (SC == SC_Extern || SC == SC_PrivateExtern))
+  if (ND->isExternC() && (SC == SC_Extern || SC == SC_PrivateExtern))
     return true;
   return ND->getDeclContext()->isTranslationUnit();
 }
@@ -6581,7 +6581,7 @@
     // If this function is declared as being extern "C", then check to see if 
     // the function returns a UDT (class, struct, or union type) that is not C
     // compatible, and if it does, warn the user.
-    if (NewFD->hasCLanguageLinkage()) {
+    if (NewFD->isExternC()) {
       QualType R = NewFD->getResultType();
       if (R->isIncompleteType() && !R->isVoidType())
         Diag(NewFD->getLocation(), diag::warn_return_value_udt_incomplete)