Reland: [OpenMP][clang] 6.0: num_threads strict (part 1: host runtime) (#147532)

OpenMP 6.0 12.1.2 specifies the behavior of the strict modifier for the
num_threads clause on parallel directives, along with the message and
severity clauses. This commit implements necessary host runtime changes.

Reland https://github.com/llvm/llvm-project/pull/146403. After manual
testing on a gfx90a machine, I could not reproduce the failing test,
which makes it even more likely that the test has just been flaky. (Or
at least that it's not an issue related to this patch.)

openmp/runtime/src/kmp_runtime.cpp

diff --git a/openmp/runtime/src/kmp_runtime.cpp b/openmp/runtime/src/kmp_runtime.cpp
index 417eceb..6afea9b 100644
--- a/openmp/runtime/src/kmp_runtime.cpp
+++ b/openmp/runtime/src/kmp_runtime.cpp
@@ -1214,6 +1214,12 @@
   // Reset for next parallel region
   this_thr->th.th_set_proc_bind = proc_bind_default;
 
+  // OpenMP 6.0 12.1.2 requires the num_threads 'strict' modifier to also have
+  // effect when parallel execution is disabled by a corresponding if clause
+  // attached to the parallel directive.
+  if (this_thr->th.th_nt_strict && this_thr->th.th_set_nproc > 1)
+    __kmpc_error(this_thr->th.th_nt_loc, this_thr->th.th_nt_sev,
+                 this_thr->th.th_nt_msg);
   // Reset num_threads for next parallel region
   this_thr->th.th_set_nproc = 0;