Don't use comdats for initializers on platforms that don't support it

In particular, pre-.init_array ELF uses the .ctors section mechanism.
MinGW COFF also uses .ctors, now that I think about it. Therefore,
restrict this optimization to the two platforms that are currently known
to work: ELF with .init_array and COFF with .CRT$XCU.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218287 91177308-0d34-0410-b5e6-96231b3b80d8