[CMake] Enable warnings for runtimes

[CMake] Enable warnings for runtimes

Summary:
`HandleLLVMOptions` adds `-w` to the cflags if `LLVM_ENABLE_WARNINGS` is not on.
With `-w`, `check_cxx_compiler_flag` doesn't error out for unsupported flags
(for example `-mcrc` on x86_64), and those flags end up being detected as
working - and really they aren't.

I am not entirely sure what the best way to solve this is, but setting
`LLVM_ENABLE_WARNINGS` prior to including `HandleLLVMOptions` does the job.

Reviewers: phosek, beanz

Reviewed By: phosek

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46079

llvm-svn: 331042