clang/Lex: Stop using SourceManager::getBuffer

Update clang/lib/Lex to stop relying on a `MemoryBuffer*`, using the
`MemoryBufferRef` from `getBufferOrNone` since both locations had logic
for checking validity of the buffer. There's potentially a functionality
change, since the logic was wrong (it checked for `nullptr`, which was
never returned by the old API), but if that was reachable the new
behaviour should be better.

Differential Revision: https://reviews.llvm.org/D89402