clang/Frontend: Use MemoryBufferRef in FrontendInputFile (and remove SourceManager::getBuffer)

In order to drop the final callers to `SourceManager::getBuffer`, change
`FrontendInputFile` to use `Optional<MemoryBufferRef>`. Also updated
the "unowned" version of `SourceManager::createFileID` to take a
`MemoryBufferRef` (it now calls `MemoryBuffer::getMemBuffer`, which
creates a `MemoryBuffer` that does not own the buffer data).

Differential Revision: https://reviews.llvm.org/D89427