[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 +631,9 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses dangling references for C++20's parenthesized aggregate initialization (#101957).
 
+- A new off-by-default warning ``-Wms-bitfield-compatibility`` has been added to alert to cases where bit-field
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ojhunt wrote:

I think we'll probably want a general MSVC abi compatibility warning that encompasses this PR, the above alignment example, the signed nonsense, etc for projects that want to have some kind of compatibility with out using the various force-msvc-abi flags on their projects

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