clang/{Format,Rewrite}: Stop using SourceManager::getBuffer, NFC

Update clang/lib/Format and clang/lib/Rewrite to use a `MemoryBufferRef`
from `getBufferOrFake` instead of `MemoryBuffer*` from `getBuffer`.

No functionality change here, since the call sites weren't checking if
the buffer was valid.

Differential Revision: https://reviews.llvm.org/D89406