merge hasCLanguageLinkage and isExternC. Keep the shorter name.

I added hasCLanguageLinkage while fixing some language linkage bugs some
time ago so that I wouldn't have to check all users of isExternC. It turned
out to be a much longer detour than expected, but this patch finally
merges the two again. The isExternC function now implements just the
standard notion of having C language linkage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175119 91177308-0d34-0410-b5e6-96231b3b80d8