[NFC] Factor out + document build requirements

Summary: This change factors out compiler checking / warning, and documents LLVM_FORCE_USE_OLD_TOOLCHAIN. It doesn't introduce any functional changes nor policy changes, these will come late.

Subscribers: mgorny, jkorous, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D56799

llvm-svn: 351387