Enable ninja by default on MSVC #43767
Description
When working locally dmajor has found that apparently disabling LLVM assertions doesn't work on MSVC when compiling LLVM. Despite -DLLVM_ASSERTIONS=OFF apparently they're still compiled in!
This isn't the first time IIRC of LLVM definitions and such not actually getting communicated to the msbuild build system. My guess is that this is a bug in LLVM's build system or something like that. In any case I think we should at least start using ninja by default if we detect ninja in PATH, and otherwise we may want to print warnings on "known bad" cases saying that msbuild isn't reading configuration options correctly, recommending a download of ninja