lit: Fix a subtle resource usage bug when executing tests using the internal

shell runner.

We would inadvertently end up holding on to handles to the temporary files
longer than we should have been. On Win32, where open handles lock some file
operations, this caused problems in tests which would try to move temporary
files around (as Clang does by default now).

Many thanks to Francois Pichet for the excellent detective work on this.

llvm-svn: 115040