IR: Aliases don't belong to an explicit comdat

Aliases inherit their comdat from their aliasee, they don't have an
explicit comdat.

This fixes PR20279.
Fix clang tests to be compliant with LLVM IR aliases

Comdat IR references were mistakenly printed for aliases when they
passed through the IR/AsmWriter code.

This makes clang's tests not check for the existance of these wrongly
printed comdat references.