[clang] [clang][docs] Add note about `-Wms-bitfield-padding` in MSVC compatibility docs (PR #155770)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt updated https://github.com/llvm/llvm-project/pull/155770

>From cc25a94f6f2e28e386b00ca8feee4d2c53c1ee01 Mon Sep 17 00:00:00 2001
From: Oliver Hunt <oliver at apple.com>
Date: Thu, 28 Aug 2025 00:01:13 -0700
Subject: [PATCH] [clang][docs] Add note about `-Wms-bitfield-padding` in MSVC
 compatibility docs

---
 clang/docs/MSVCCompatibility.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/clang/docs/MSVCCompatibility.rst b/clang/docs/MSVCCompatibility.rst
index b4a7d23e1b2c6..63c8d8e70a997 100644
--- a/clang/docs/MSVCCompatibility.rst
+++ b/clang/docs/MSVCCompatibility.rst
@@ -284,3 +284,9 @@ Since `__m128` is not a class type in clang any overloads after a template defin
 With MSVC ``foo(__m128)`` will be selected but with clang ``foo<__m128>()`` will be selected since on clang `__m128` is a builtin type.
 
 In general the takeaway is `__m128` is a builtin type on clang while a class type on MSVC.
+
+Warnings
+========
+
+* ``-Wms-bitfield-padding``
+  When generating code compatible with MSVC, clang applies the bit-field padding ABI used by MSVC. The padding behavior of sequential bit-fields in a record's layout is dependent on whether the underlying storage type of those bit-fields are the same. To help diagnose unexpected padding the ``-Wms-bitfield-padding`` warning can be used to diagnose cases where the MSVC ABI will not pad bit-fields (even if not targeting relevant platforms).