[XRay] Guard call to postCurrentThreadFCT()

Summary:
Some cases where `postCurrentThreadFCT()` are not guarded by our
recursion guard. We've observed that sometimes these can lead to
deadlocks when some functions (like memcpy()) gets outlined and the
version of memcpy is XRay-instrumented, which can be materialised by the
compiler in the implementation of lower-level components used by the
profiling runtime.

This change ensures that all calls to `postCurrentThreadFCT` are guarded
by our thread-recursion guard, to prevent deadlocks.

Reviewers: mboerger, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53805

llvm-svn: 345489