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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181163 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed