Fix stepping a virtual thread when the python operating system was enabled.

Fix stepping a virtual thread when the python operating system was enabled.

The OperatingSystem plug-ins allow code to detect threads in memory and then say "memory thread 0x11111" is backed by the actual thread 1.

You can then single step these virtual threads. A problem arose when thread specific breakpoints were used during thread plans where we would say "set a breakpoint on thread 0x11111" and we would hit the breakpoint on the real thread 1 and the thread IDs wouldn't match and we would get rid of the "stopped at breakpoint" stop info due to this mismatch. Code was added to ensure these events get forwarded and thus allow single stepping a memory thread to work correctly.

Added a test case for this as well.

<rdar://problem/19211770>

llvm-svn: 234364