[clang] [CIR] Upstream initial support for union type (PR #137501)

[Andy Kaylor via cfe-commits]

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@@ -306,3 +320,71 @@ CIRGenTypes::computeRecordLayout(const RecordDecl *rd, cir::RecordType *ty) {
   // TODO: implement verification
   return rl;
 }
+
+void CIRRecordLowering::lowerUnion() {
+  CharUnits layoutSize = astRecordLayout.getSize();
+  mlir::Type storageType = nullptr;
+  bool seenNamedMember = false;
+
+  // Iterate through the fields setting bitFieldInfo and the Fields array. Also
+  // locate the "most appropriate" storage type.  The heuristic for finding the
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andykaylor wrote:

I'm not sure what any of this comment after "locate the 'most appropriate' storage type" means. The comment was carried over from the classic codegen, but I wonder if it means we're doing a lot of work here that we don't need to do.

https://github.com/llvm/llvm-project/pull/137501