Make InferWidths thread safe (#1775) (#1779)

Force a synchronization point on InferWidths.run due to get a lock on
the mutable constraint solver. This prevents a rare threading issue
where the same compiler is run multiple times in parallel.

This is a backport of #1775 using a different technique that is binary
compatible with the 1.3.x.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>