[IR] Allow available_externally GlobalAlias

GlobalVariable and Function can be available_externally. GlobalAlias is used
similarly. Allowing available_externally is a natural extension and helps
ThinLTO discard GlobalAlias in a non-prevailing COMDAT (see D135427).

For now, available_externally GlobalAlias must point to an
available_externally GlobalValue (not ConstantExpr).

Differential Revision: https://reviews.llvm.org/D137441