Fix crash with UniqueExternalLinkage in extern "C" contexts

[Rafael Espíndola]

Clang currently hits the assert in GetGVALinkageForVariable when compiling

namespace {
  struct X {};
}
extern "C" {
  X b = X();
}

The attached patch fixes it by propagating the linkage to variables,
even if they are extern "C". This seems consistent with how we handle
other interactions of language linkage and non-external linkage.

Cheers,
Rafael
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