Make PrettyStackTraceEntry use ManagedStatic for its ThreadLocal.

Make PrettyStackTraceEntry use ManagedStatic for its ThreadLocal.

This was somewhat tricky because ~PrettyStackTraceEntry() may run after
llvm_shutdown() has been called. This is rare and only happens for a common idiom
used in the main() functions of command-line tools. This works around the idiom by
skipping the stack clean-up if the PrettyStackTraceHead ManagedStatic is not
constructed (i.e. llvm_shutdown() has been called).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190730 91177308-0d34-0410-b5e6-96231b3b80d8