[clang] [FMV] Allow fmv without default declaration. (PR #85454)

[Alexandros Lamprineas via cfe-commits]

labrinea wrote:

Hmm, I am now having second thoughts about this change. All is well without a default declaration as long as we don't have a caller in the TU. In that case (if there is a caller) a resolver is needed, but without a default declaration we can't create a complete resolver (the resolver contains calls to the target versions it can see in the TU) :/

https://github.com/llvm/llvm-project/pull/85454