Fix nested CrashRecoveryContexts with LLVM_ENABLE_THREADS=OFF, allow them.

libclang uses a CrashRecoveryContext, and building a module does too. If a
module gets built through libclang, nested CrashRecoveryContexts are used.  They
work fine with threads as things are stored in ThreadLocal variables, but in
LLVM_ENABLE_THREADS=OFF builds the two recovery contexts would write to the
same globals.

To fix, keep active CrashRecoveryContextImpls in a list and have the global
point to the innermost one, and do something similar for
tlIsRecoveringFromCrash.

Necessary (but not sufficient) for PR11974 and PR20325

http://reviews.llvm.org/D11770

llvm-svn: 244251