[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:
http://lists.llvm.org/pipermail/openmp-dev/2019-August/002668.html

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

llvm-svn: 368999