[clang/CachedDiagnostics] Fix issues with how scratch buffers are handled

* Use the same memory buffer of the scratch buffer for round-tripping, instead of copying it
and creating multiple FileIDs for the same buffer.
* Make sure the round-tripped scratch buffer uses updated line table.
* Using a `MemoryBuffer` for source buffers also ensures null termination.

rdar://106090263
(cherry picked from commit 000a48f)

Author: akyrtzi