Add {ExternalSemaSource,SemaConsumer}::ForgetSema callback, and update PCHReader
to use it so it at least won't try to access Sema once it is gone.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90261 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed