[libFuzzer] Guard symbolization with try-lock.

Summary:
When out-of-memory or timeout occurs, threads can be stopped during
symbolization, thereby causing a deadlock when the OOM/TO handlers
attempt symbolization.  We avoid this deadlock by skipping symbolization
if another thread is symbolizing.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46605

llvm-svn: 331825