When checking the namespace of a redeclaration or definition, look through linkage specs. Fixes PR5430.

llvm-svn: 86461
diff --git a/clang/test/SemaCXX/linkage-spec.cpp b/clang/test/SemaCXX/linkage-spec.cpp
index 53cd61c..fc9b3ab 100644
--- a/clang/test/SemaCXX/linkage-spec.cpp
+++ b/clang/test/SemaCXX/linkage-spec.cpp
@@ -33,3 +33,10 @@
     };
   }
 }
+
+// PR5430
+namespace pr5430 {
+  extern "C" void func(void);
+}
+using namespace pr5430;
+extern "C" void pr5430::func(void) { }