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

[Iris Shi via cfe-commits]

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@@ -5,25 +5,146 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t.ll
 // RUN: FileCheck --check-prefix=OGCG --input-file=%t.ll %s
 
+union U1 {
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el-ev wrote:

Padded and packed unions are added, `CIRGenTypes::isZeroInitializable` doesn't contain a path that returns false without triggering errorNYI.

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