OpenMP/C++: Fix declare_variant's 'adjust_args' if there is a 'this' pointer [PR118321]

OpenMP/C++: Fix declare_variant's 'adjust_args' if there is a 'this' pointer [PR118321]

The adjust_args clause is internally store as the i-th argument to the function,
which fails if hidden arguments come before. This commit handles the C++ 'this'
pointer by shifting the internal arg index by one.

	PR fortran/118321

gcc/cp/ChangeLog:

	* decl.cc (omp_declare_variant_finalize_one): Shift adjust_args index
	by one for non-static class function's 'this' pointer.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/adjust-args-4.C: New test.