Explore using LLVM on Windows instead of MSVC

[dktapps]

Legend has it that LLVM is better for performance and is less quirky than MSVC.

[dktapps]

As it turns out, LLVM is not well supported by php-src: php/php-src#11313

This is therefore not likely to be doable in the near future (it was already low priority).

[BlackCatDevel0per]

llvm & php.. So pain... (I've compiled it a few times, but some libs couldn't be compile with llvm..)