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

[Oliver Hunt via cfe-commits]

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@@ -633,6 +633,52 @@ 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-compatibility"> {
+  code Documentation = [{
+    Under the MS Windows platform ABI, adjacent bit-fields are not packed if the
+    underlying type has a different storage size. This warning indicates that a
+    pair of adjacent bit-fields may not pack in the same way due to this behavioural
+    difference.
+
+    This can occur when mixing different types explicitly:
+
+    .. code-block:: c++
+
+      struct S {
+        uint16_t field1 : 1;
+        uint32_t field2 : 1;
+      };
+
+    or more subtly through enums
+
+    .. code-block:: c++
+
+      enum Enum1 { /* ... */ };
+      enum class Enum2 : unsigned char { /* ... */ };
+      struct S {
+        Enum1 field1 : 1;
+        Enum2 field2 : 1;
+      };
+
+    In each of these cases under the MS Windows platform ABI the second bit-field
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ojhunt wrote:

Fixed

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