[libclang] Make sure we don't try to erase past the StoredDiagnostics vector.

Ted came upon the bug but I couldn't make a test out of it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142805 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed