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

[Trevor Gross via cfe-commits]

tgross35 wrote:

I'm struggling a bit with how to handle ABI information since that affects layout (e.g. ARM aapcs), which I think explains most of the errors in https://buildkite.com/llvm-project/github-pull-requests/builds/31198#018d26e2-fd17-4e15-a1eb-08580c189056. This needs to be available at TargetLoweringBase::InitLibcalls, which calls [`getCLayouts`](https://github.com/llvm/llvm-project/blob/cb3bf7540cf9b797575c625318e47a33f7514fad/llvm/lib/TargetParser/Triple.cpp#L1945).

TargetMachine is available at that time, so would it be better to move CLayouts from Triple to TargetMachine? If so subclasses could be used rather than the if block, which more closely follows the Clang side.

Also, are there currently any module flags that make it to TargetLowering? Looking for a reference on how get the -mlong-double-128 information.

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