Make all 'is in extern "C"' tests use the lexical context.

I was not able to find a case (other than the fix in r181163) where this
makes a difference, but it is a more obviously correct API to have.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181165 91177308-0d34-0410-b5e6-96231b3b80d8