merge hasCLanguageLinkage and isExternC

[Rafael Espíndola]

This is a followup to my previous patch and it finally merges
hasCLanguageLinkage and isExternC back (keeping the shorter name). To
the best of my knowledge hasCLanguageLinkage (now isExternC)
implements the C++ standard notions of "has C language linkage" when
in c++ mode. In C mode it classifies anything with an external linkage
as have C language linkage.

Sorry for the long detour, getting this right was a lot harder than I
was expecting.

Cheers,
Rafael
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