Fix a race condition when terminating inferiors on Windows.

If a breakpoint was hit in the inferior after shutdown had
started but before it was complete, it would cause an unclean
terminate of the inferior, leading to various problems the most
visible of which is that handles to the inferior executable would
remain locked, and the test suite would fail to run subsequent
tests because it could not recompile the inferior.

This fixes a major source of flakiness in the test suite.

llvm-svn: 247929