PR33902: Invalidate line number cache when adding more text to existing buffer.

This led to crashes as the line number cache would report a bogus line number
for a line of code, and we'd try to find a nonexistent column within the line
when printing diagnostics.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309503 91177308-0d34-0410-b5e6-96231b3b80d8