[clang] [clang] Stub out gcc_struct attribute (PR #71148)

[John McCall via cfe-commits]

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@@ -5031,7 +5031,12 @@ void RecordDecl::completeDefinition() {
 /// This which can be turned on with an attribute, pragma, or the
 /// -mms-bitfields command-line option.
 bool RecordDecl::isMsStruct(const ASTContext &C) const {
-  return hasAttr<MSStructAttr>() || C.getLangOpts().MSBitfields == 1;
+  if (hasAttr<MSStructAttr>())
+    return true;
+  if (hasAttr<GCCStructAttr>())
+    return false;
+  return C.getLangOpts().MSBitfields.value_or(
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rjmccall wrote:

Is there a reason we don't just set the lang opt correctly for the target?  I'm pretty sure language option defaults depending on target settings is well-established.

https://github.com/llvm/llvm-project/pull/71148