[UBSan] Introduce ScopedReport object.

This object is used to encapsulate all actions that need to be
done before/after printing UBSan diagnostics. Currently these
actions are:
* locking a mutex to ensure that UBSan diagnostics from several
threads won't mix with each other and with other sanitizers'
reports
* killing a program once the report is printed (if necessary).

Use this object in all UBSan handlers. Unify the way we implement
fatal and non-fatal handlers by making all the handlers simple
one-liners that redirect __ubsan_handle_foo(_abort)? to
handleFooImpl().