openmp: Add warning when functions containing metadirectives with 'construct={target...

openmp: Add warning when functions containing metadirectives with 'construct={target}' called directly

void f(void)
{
  #pragma omp metadirective \
    when (construct={target}: A) \
    default (B)
    ...
}
...
{
  #pragma omp target
    f(); // Target call

  f(); // Local call
}

With the OpenMP 5.0/5.1 specifications, we would expect A to be selected in
the metadirective when the target call is made, but B when f is called
directly outside of a target context.  However, since GCC does not have
separate copies of f for local and target calls, and the construct selector
is static, it must be resolved one way or the other at compile-time (currently
in the favour of selecting A), which may be unexpected behaviour.

This patch attempts to detect the above situation, and will emit a warning
if found.

2022-01-28  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* gimplify.c (gimplify_omp_metadirective): Mark offloadable functions
	containing metadirectives with 'construct={target}' in the selector.
	* omp-general.c (omp_has_target_constructor_p): New.
	* omp-general.h (omp_has_target_constructor_p): New prototype.
	* omp-low.c (lower_omp_1): Emit warning if marked functions called
	outside of a target context.

	gcc/testsuite/
	* c-c++-common/gomp/metadirective-4.c (main): Add expected warning.
	* gfortran.dg/gomp/metadirective-4.f90 (test): Likewise.

	libgomp/
	* testsuite/libgomp.c-c++-common/metadirective-2.c (main): Add
	expected warning.
	* testsuite/libgomp.fortran/metadirective-2.f90 (test): Likewise.