[compiler-rt] Exclude compiler-rt from using -fmodules when LLVM_ENABLE_MODULES=On

Sanitizers are intentionally not including system headers and often declare slightly different function prototypes, which is incompatible with -fmodules and -fcxx-modules. Let’s simply exclude compiler-rt from using -fmodules.

Differential Revision: https://reviews.llvm.org/D25230

llvm-svn: 283658