[OpenMP][clang] 6.0: num_threads strict (part 2: device runtime)

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 device runtime
changes.

Author: ro-i

openmp/runtime/src/kmp.h

@@ -4629,6 +4629,7 @@ extern void (*kmp_target_sync_cb)(ident_t *loc_ref, int gtid,
 #endif // ENABLE_LIBOMPTARGET
 
 // Support for error directive
+// See definition in offload/DeviceRTL DeviceTypes.h
 typedef enum kmp_severity_t {
   severity_warning = 1,
   severity_fatal = 2