Wrong LLVM tools picked up if using MSVC activation and custom LLVM build

[xhochy]

Bug report

Bug description:

For the last weeks, the MSVC installations on GitHub Actions and Azure Pipelines have also shipped their own version of LLVM tools. These don't match the ones requested in the CPython build. The MSVC activation scripts sadly put themselves in front of our local tools. While there is already code to handle the LLVM tools build via externals system on Windows or to look into the brew installation, there is no explicit override existing.

CPython versions tested on:

CPython main branch, 3.13, 3.14

Operating systems tested on:

Windows

Linked PRs

• gh-138451: Support custom LLVM installation path #138452