ContentCache: Simplify by always owning the MemoryBuffer

This changes `ContentCache::Buffer` to use
`std::unique_ptr<MemoryBuffer>` instead of the `PointerIntPair`. It
drops the (mostly unused) `DoNotFree` bit, instead creating a (new)
non-owning `MemoryBuffer` instance when passed a `MemoryBufferRef`.

Differential Revision: https://reviews.llvm.org/D67030