OpenMP: Unshare expr in context-selector condition [PR121922]

As the testcase shows, a missing unshare_expr caused that the condition
was only evaluated once instead of every time when a 'declare variant'
was resolved.

	PR middle-end/121922

gcc/ChangeLog:

	* omp-general.cc (omp_dynamic_cond): Use 'unshare_expr' for
	the user condition.

libgomp/ChangeLog:

	* testsuite/libgomp.c-c++-common/declare-variant-1.c: New test.

Co-authored-by: Sandra Loosemore <sloosemore@baylibre.com>
(cherry picked from commit 97c1d2fa97afc59b02d2894b88069136145603e2)