[OpenMP] Fix race condition in omp_init_lock
This is a partial fix for bug 34050.
This prevents callers of omp_set_lock (which does not hold kmp_global_lock)
from ever seeing an uninitialized version of kmp_i_lock_table.table.
It does not solve a use-after-free race condition if omp_set_lock obtains a
pointer to __kmp_i_lock_table.table before it is updated and then attempts to
dereference afterwards. That race is far less likely and can be handled in a
The unit test usually segfaults on the current trunk revision. It passes with
Patch by Adam Azarchs
Differential Revision: https://reviews.llvm.org/D39439