Lexer: Update the Lexer to use MemoryBufferRef, NFC

Update `Lexer` / `Lexer::Lexer` to use `MemoryBufferRef` instead of
`MemoryBuffer*`. Callers that were acquiring a `MemoryBuffer*` via
`SourceManager::getBuffer` were updated, such that if they checked
`Invalid` they use `getBufferOrNone` and otherwise `getBufferOrFake`.

Differential Revision: https://reviews.llvm.org/D89398