[clang] [llvm] [ARM] Add support for Windows SEH (PR #184953)

[Trung Nguyen via cfe-commits]

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@@ -1110,6 +1123,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF,
     DefCFAOffsetCandidates.addInst(LastPush, GPRCS3Size, BeforeFPPush);
     if (FramePtrSpillArea == SpillArea::GPRCS3)
       BeforeFPPush = false;
+    NumBytes -= GPRCS3Size;
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trungnt2910 wrote:

This change is required to make indexing variables from `R6` work for functions that would otherwise use `R11`.

Without this, `R6 + (MFI.getObjectOffset(FI) + MFI.getStackSize())` would always be 8 bytes off because the original SP was subtracted 8 bytes more than needed. 

https://github.com/llvm/llvm-project/pull/184953