Avoid a race condition in opt-viewer/optrecord (#131214)
See https://bugzilla.redhat.com/2336915
See https://reviews.llvm.org/D41784?id=
See
https://github.com/androm3da/optviewer-demo/issues/4#issuecomment-718787822
Fixes https://github.com/llvm/llvm-project/issues/62403.
The race condition happened when the demangler_proc was being set. The
locking mechanism itself happened too late.
This way, the lock always exists (to avoid a race when creating it) and
is always used when *creating* demangler_proc.
