Support: Add operator== for MemoryBufferRef and split out MemoryBufferRef.h

As preparation for changing `LineIterator` to work with `MemoryBufferRef`:

- Add an `operator==` that uses buffer pointer identity to ensure two buffers
  are equivalent.
- Split out `MemoryBufferRef.h`, to avoid polluting `LineIterator.h` includers
  with everything from `MemoryBuffer.h`. This also means moving the
  `MemoryBuffer` constructor to a source file.

Differential Revision: https://reviews.llvm.org/D89279