gn build: Build clang with -fno-strict-aliasing, make building with gcc much quieter

- gcc doesn't understand -Wstring-conversion, so pass that only to clang
- disable a few gcc warnings that are noisy and also disabled in the cmake build
- -Wstrict-aliasing pointed out that the cmake build builds clang with
  -fno-strict-aliasing, so do that too

Differential Revision: https://reviews.llvm.org/D57191

llvm-svn: 352141