[clang] [Clang][AST] Fix MS Mangle concept uneval context template instantiation crash (PR #117845)

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+// RUN: %clang_cc1 -std=c++20 -fms-compatibility -fms-compatibility-version=19.33 -emit-llvm %s -o - -triple=x86_64-windows-msvc | FileCheck %s
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zmodem wrote:

I think doing codegen (-emit-llvm) in clang/test/AST/ is unusual, those tests normally just look at the AST (though there are exceptions). Maybe there is a better place for this?

https://github.com/llvm/llvm-project/pull/117845