Fix TestThreadSpecificBreakpoint on Windows

This test was frequently hanging on Windows, causing a timeout after
10 minutes.  The short delay (100 microsecond) in the sample program
could cause a deadlock in the Windows thread pool, as I've explained
in the test program's comments.

Now that it doesn't hang, it passes reliably, so I've removed the
Windows-specific XFAIL.

I've tried to clarify the comments in TestThreadSpecificGBreakpoint.py
by eliminating some redundancy and typos, and I simplified away a
couple unnecessary assignments.

Differential Revision: https://reviews.llvm.org/D65546

llvm-svn: 367573