[ARM] Use SEH exception handling on MS

[martell]

Summary

The correct value is already set for aarch64.
We have just never set this for arm.

https://docs.microsoft.com/en-us/cpp/build/arm-exception-handling

[mstorsjo]

I remember @compnerd saying that the SEH/ARM was documented but not actually really implemented yet: https://reviews.llvm.org/D39532#914461

[martell]

In D40286#931254, @mstorsjo wrote:

I remember @compnerd saying that the SEH/ARM was documented but not actually really implemented yet: https://reviews.llvm.org/D39532#914461

Is SEH implemented for aarch64 because if not then I would prefer setting SEH here like we do for aarch64?

[mstorsjo]

In D40286#931319, @martell wrote:

Is SEH implemented for aarch64 because if not then I would prefer setting SEH here like we do for aarch64?

No, it isn't. So if you try to compile code in msvc mode that uses exceptions, it'll fail due to unimplemented codepaths. It was set/enabled on aarch64 as a side effect in SVN r308665 (git mirror commit 8c714df517f9a42f5b360b2a2ca619facd961b40).

But consistency obviously is nice, and this is what would need to be implemented for exceptions for msvc mode for arm in any case, so I don't mind setting it - the failures make it much clearer that the exceptions actually don't work yet.

llvm/trunk/lib/Target/ARM/MCTargetDesc/ARMMCAsmInfo.cpp

if (!UseIntegratedAssembler) {
     DwarfRegNumForCFI = true;
   }
 }
 
 void ARMCOFFMCAsmInfoMicrosoft::anchor() { }
 
 ARMCOFFMCAsmInfoMicrosoft::ARMCOFFMCAsmInfoMicrosoft() {
   AlignmentIsInBytes = false;
-
+  ExceptionsType = ExceptionHandling::WinEH;
   PrivateGlobalPrefix = "$M";
   PrivateLabelPrefix = "$M";
   CommentString = ";";
 }
 
 void ARMCOFFMCAsmInfoGNU::anchor() { }
 
 ARMCOFFMCAsmInfoGNU::ARMCOFFMCAsmInfoGNU() {