[CMake] [MinGW] Enable use of LLVM_USE_SANITIZER in a MinGW environment

Currently using LLVM_USE_SANITIZER with a MinGW target leads to a fatal
configuration error due to an unsupported platform. MinGW targets on
clang however implement a few sanitizers, currently ASAN and UBSAN.

This patch enables LLVM_USE_SANITIZER in a MinGW environment as well.

Differential Revision: https://reviews.llvm.org/D95750