[Frontend] Simplify PrecompiledPreamble::PCHStorage. NFC

- Remove fiddly union, preambles are heavyweight
- Remove fiddly move constructors in TempPCHFile and PCHStorage, use unique_ptr
- Remove unneccesary accessors on PCHStorage
- Remove trivial InMemoryStorage
- Move implementation details into cpp file

This is a prefactoring, followup change will change the in-memory PCHStorage to
avoid extra string copies while creating it.

Differential Revision: https://reviews.llvm.org/D124177