Make the error-handling functions thread-safe.

Prior to this change, error handling functions must be installed
and removed only inside of an llvm_[start/stop]_multithreading
pair.  This change allows error handling functions to be installed
any time, and from any thread.

Reviewed by: chandlerc

Differential Revision: http://reviews.llvm.org/D4140

llvm-svn: 210937