LLVM build module fixes: - Avoid jumping to non-existent labels; - fix logic used to decide when Visual Studio environment needs to be programatically loaded; - Meson build stub: Avoid a situation where Meson build is attempted.
LLVM build module fixes:
- Avoid jumping to non-existent labels;
- fix logic used to decide when Visual Studio environment needs to be programatically loaded;
- Meson build stub: Avoid a situation where Meson build is attempted.

pal1000/mesa-dist-win,pal1000/mesa-dist-win