Reject overloading of two static extern C functions.

This patch moves hasCLanguageLinkage to be VarDecl and FunctionDecl methods
so that they can be used from SemaOverload.cpp and then fixes the logic
in Sema::IsOverload.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171193 91177308-0d34-0410-b5e6-96231b3b80d8