clang-{tools,unittests}: Stop using SourceManager::getBuffer, NFC

Update clang-tools-extra, clang/tools, clang/unittests to migrate from
`SourceManager::getBuffer`, which returns an always dereferenceable
`MemoryBuffer*`, to `getBufferOrNone` or `getBufferOrFake`, both of
which return a `MemoryBufferRef`, depending on whether the call site was
checking for validity of the buffer. No functionality change intended.

Differential Revision: https://reviews.llvm.org/D89416