Use lexical contexts when checking for conflicting language linkages.

This fixes pr14958. I will audit other calls to isExternCContext to see
if there are any similar bugs left.

llvm-svn: 181163
diff --git a/clang/test/SemaCXX/linkage-spec.cpp b/clang/test/SemaCXX/linkage-spec.cpp
index 0ba9508..504df0d 100644
--- a/clang/test/SemaCXX/linkage-spec.cpp
+++ b/clang/test/SemaCXX/linkage-spec.cpp
@@ -106,3 +106,11 @@
     return sizeof(ArtsSink);
   }
 }
+
+namespace pr14958 {
+  namespace js { extern int ObjectClass; }
+  extern "C" {
+    namespace js {}
+  }
+  int js::ObjectClass;
+}