[clang] Workaround for MSVC ARM64 build performance regression (PR #65215)

[Tobias Hieta via cfe-commits]

https://github.com/tru requested changes to this pull request.

Thanks for the PR, I think we at least need to make sure that we don't erase flags that have been set by the user manually.

Another option would just to say that this configuration ARM64 + MSVC of this version is not a valid configuration for MSVC and add it to ProblematicConfigurations here: https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/CheckProblematicConfigurations.cmake

https://github.com/llvm/llvm-project/pull/65215