[llvm] [clang] [WIP] Correct lowering of `fp128` intrinsics (PR #76558)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

For the question about querying module flags, we do that in a few different places in codegen; grep for "getModuleFlag".  Not sure if there's anything specifically in TargetLowering.

https://github.com/llvm/llvm-project/pull/76558