[windows] Always pass fp128 arguments indirectly (#128848)

LLVM currently expects `__float128` to be both passed and returned in
xmm registers on Windows. However, this disagrees with the Windows
x86-64 calling convention [1], which indicates values larger than 64
bits should be passed indirectly.

Update LLVM's default Windows calling convention to pass `fp128`
directly.  Returning in xmm0 is unchanged since this seems like a
reasonable extrapolation of the ABI. With this patch, the calling
convention for `i128` and `f128` is the same.

GCC passes `__float128` indirectly, which this also matches. However, it
also returns indirectly, which is not done here. I intend to attempt a
GCC change to also return in `xmm0` rather than making that change here,
given the consistency with `i128`.

This corresponds to the frontend change in [2], see more details there.

[1]:
https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170
[2]: #115052

Author: tgross35

llvm/docs/ReleaseNotes.md

@@ -122,6 +122,9 @@ Changes to the WebAssembly Backend
 Changes to the Windows Target
 -----------------------------
 
+* `fp128` is now passed indirectly, meaning it uses the same calling convention
+  as `i128`.
+
 Changes to the X86 Backend
 --------------------------
 

llvm/lib/Target/X86/X86CallingConv.td

@@ -639,8 +639,9 @@ def CC_X86_Win64_C : CallingConv<[
   // 512 bit vectors are passed by pointer
   CCIfType<[v64i8, v32i16, v16i32, v32f16, v32bf16, v16f32, v8f64, v8i64], CCPassIndirect<i64>>,
 
-  // Long doubles are passed by pointer
+  // Float types larger than 64-bits (long double and fp128) are passed by pointer
   CCIfType<[f80], CCPassIndirect<i64>>,
+  CCIfType<[f128], CCPassIndirect<i64>>,
 
   // If SSE was disabled, pass FP values smaller than 64-bits as integers in
   // GPRs or on the stack.