Add a getLanguageLinkage method to VarDecls and FunctionDecls. Use it to fix
some cases where functions with no language linkage were being treated as having
C language linkage. In particular, don't warn in

extern "C" {
  static NonPod foo();
}

Since getLanguageLinkage checks the language linkage, the linkage computation
cannot use the language linkage. Break the loop by checking just the context
in the linkage computation.