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

[Oliver Hunt via cfe-commits]

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@@ -19213,6 +19213,29 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
 
     if (Record && FD->getType().isVolatileQualified())
       Record->setHasVolatileMember(true);
+    auto IsNonDependentBitField = [](const FieldDecl *FD) {
+      if (!FD->isBitField())
+        return false;
+      if (FD->getType()->isDependentType())
+        return false;
+      return true;
+    };
+
+    if (Record && PreviousField && IsNonDependentBitField(FD) &&
+        IsNonDependentBitField(*PreviousField)) {
+      unsigned FDStorageSize =
+          Context.getTypeSizeInChars(FD->getType()).getQuantity();
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ojhunt wrote:

@rnk eep, thanks! I think the reason I switched to getQuantity was because diagprinter can't handle char units (I may just add that in a later patch as it seems like an obvious thing to support)

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