Frontend: Stop leaking when not -disable-free

Try again to plug a leak that's been around since at least r128011 after
coming across the FIXME.  Nico Weber tried something similar in r207065
but had to revert in r207070 due to a bot failure.

The build failure isn't visible anymore so I'm not sure what went wrong.
I'm doing this slightly differently -- when not -disable-free I'm still
resetting the members (just not leaking them) -- so maybe it will work
out this time?  Tests pass locally, anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236419 91177308-0d34-0410-b5e6-96231b3b80d8