Delete temporary output files when an error occurs during PCH reading.

This reduces the spam make test leaves behind in /tmp. The assert isn't
particularly useful because it's not run with -disable-free (the default when
using the clang driver) but should cover all -cc1 tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165910 91177308-0d34-0410-b5e6-96231b3b80d8