[clang] Add an off-by-default warning to complain about MSVC bitfield padding (PR #117428)

[Oliver Hunt via cfe-commits]

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@@ -631,6 +631,7 @@ def Packed : DiagGroup<"packed", [PackedNonPod]>;
 def PaddedBitField : DiagGroup<"padded-bitfield">;
 def Padded : DiagGroup<"padded", [PaddedBitField]>;
 def UnalignedAccess : DiagGroup<"unaligned-access">;
+def MSBitfieldCompatibility : DiagGroup<"ms-bitfield-packing-compatibility">;
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ojhunt wrote:

I've removed -packing- because I could see us extending this to also warn in cases where the ms "signed by default" nonsense could cause issues - e.g. rather than a new flag for every possible scenario we can just have one.

https://github.com/llvm/llvm-project/pull/117428