[clang][SLH] Add __has_feature(speculative_load_hardening)

SLH doesn't support asm goto and is unlikely to ever support it. Users of asm
goto need a way to choose whether to use asm goto or fallback to an SLH
compatible code path when SLH is enabled. This feature flag will give users
this ability.

Tested via unit test

Reviewed By: mattdr

Differential Revision: https://reviews.llvm.org/D79733