Pull in r309503 from upstream clang trunk (by Richard Smith):
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.
This fixes an assertion when building the graphics/champlain port.
Reported by: antoine, kwm
PR: 219139