[OpenMP] Fix detecting warning options for GCC

[OpenMP] Fix detecting warning options for GCC

If testing for a warning option like -Wno-<foo> with GCC, GCC won't
print any diagnostic at all, leading to the options being accepted
incorrectly. However later, if compiling a file that actually prints
another warning, GCC will also print warnings about these -Wno-<foo>
options being unrecognized.

This avoids warning spam like this, for every OpenMP source file that
produces build warnings with GCC:

    cc1plus: warning: unrecognized command line option ‘-Wno-int-to-void-pointer-cast’
    cc1plus: warning: unrecognized command line option ‘-Wno-return-type-c-linkage’
    cc1plus: warning: unrecognized command line option ‘-Wno-covered-switch-default’
    cc1plus: warning: unrecognized command line option ‘-Wno-enum-constexpr-conversion’

This matches how such warning options are detected and added in
llvm/cmake/modules/HandleLLVMOptions.cmake, e.g. like this:

    check_cxx_compiler_flag("-Wclass-memaccess" CXX_SUPPORTS_CLASS_MEMACCESS_FLAG)
    append_if(CXX_SUPPORTS_CLASS_MEMACCESS_FLAG "-Wno-class-memaccess" CMAKE_CXX_FLAGS)

This also matches how LLDB warning options were restructured for
GCC compatibility in e546bbfda0ab91cf78c096d8c035851cc7c3b9f3.

Differential Revision: https://reviews.llvm.org/D139922