[CMake] Disable GCC -Wnonnull

I noticed during the build that GCC would emit a ton of nonnull
warnings.

Example:
```
/usr/local/google/home/fmzakari/code/github.com/llvm/llvm-project/clang/include/clang/AST/ExternalASTSource.h:378:54: warning: ‘this’ pointer is null [-Wnonnull]
  378 |       Ptr = reinterpret_cast<uint64_t>((Source->*Get)(Ptr >> 1));
      |                                        ~~~~~~~~~~~~~~^~~~~~~~~~
```

Reviewed By: MaskRay, rsmith

Differential Revision: https://reviews.llvm.org/D155857