Make gCrashRecoveryEnabled thread local

If context is enabled/disabled and queried concurrently then this
results in a data race/TSAN failure with RunSafely (where boolean
variable was not locked).

There doesn't seem to be a reasonable way to enable threads that enable
and disable recovery in parallel (without also keeping
gCrashRecoveryEnabled's lock held during Fn execution which seems
undesirable). This makes enable checking if enabled thread local and
consistent with other thread local usage of crash context here.

Differential Revision: https://reviews.llvm.org/D93907