CrashRecovery: Add CrashRecoveryContext::GetCurrent(), so clients can find the active context from anywhere.

llvm-svn: 111308

llvm-svn: 111307

llvm-svn: 109872

llvm-svn: 109752

llvm-svn: 109721

 - This works, but won't handle crashes on stack overflow, or signals delivered
   to a thread other than the one that crashed. The latter is particular annoying
   on Darwin, because SIGABRT tends to go to the main thread.

llvm-svn: 109717

 - Designed as a simple wrapper to allow clients to attempt to catch crashes
   (memory errors, assertion violations, etc.) and do some kind of recovery.

 - Currently doesn't actually attempt to catch crashes.

llvm-svn: 109586