[clang] Revert "[clang] Remove #undef alloca workaround" (PR #81649)

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zeroomega wrote:

> for my information, which version of Visual Studio are you using?

To be precise, we don't use (not installed) full Visual Studio. We directly install Visual Studio build tools from VS2022.
The MSVC version string is `14.34.31933` with Windows Kit `10.0.19041.0`


https://github.com/llvm/llvm-project/pull/81649