Lex: Avoid MemoryBuffer* key in ExcludedPreprocessorDirectiveSkipMapping, NFC

This is a prep patch for changing SourceManager to return
`Optional<MemoryBufferRef>` instead of `MemoryBuffer`. With that change the
address of the MemoryBuffer will be gone, so instead use the start of the
buffer as the key for this map.

No functionality change intended, as it's expected that the pointer identity
matches between the buffers and the buffer data.

Radar-Id: rdar://70139990
Differential Revision: https://reviews.llvm.org/D89136