Don't delete lsan thread-local data until it's no longer required

Summary:
The routines for thread destruction in the thread registry require
the lsan thread index, which is stored in pthread tls on OS X.
This means that we need to make sure that the lsan tls isn't destroyed
until after the thread registry tls. This change ensures that we
don't delete the lsan tls until we've finished destroying the thread
in the registry, ensuring that the destructor for the lsan tls runs
after the destructor for the thread registry tls.

This patch also adds a check to ensure that the thread ID is valid before
returning it in GetThreadID(), to ensure that the above behavior
is working correctly.

Reviewers: dvyukov, kubamracek, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31884

llvm-svn: 299978