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
Duncan P. N. Exon Smith