The two libclang crash-recovery tests that involve precompiled

preambles end up leaving the precompiled preambles around. This is by
design, since we do minimal cleanup during crash recovery. However,
it's unfortunate for testing, so introduce a hook that allows these
two tests to put the precompiled preamble somewhere where we can
delete them after testing.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@113698 91177308-0d34-0410-b5e6-96231b3b80d8