[OpenMP] Turn on -Wall compiler warnings by default


[OpenMP] Turn on -Wall compiler warnings by default

Instead, maintain a list of disabled options to still build libomp and
libomptarget without warnings. This includes -Wno-error and -Wno-pedantic
to silence warnings that LLVM enables when building in-tree.

I tested the following compilers:

  • Clang 6.0, 7.0, 8.0
  • GCC 4.8.5 (CentOS 7), GCC 6, 7, 8, 9
  • Intel Compiler 16, 17, 18, 19

RFC thread on openmp-dev mailing list:

Differential Revision: https://reviews.llvm.org/D65867