[libclang] Add index option to store preambles in memory

This commit allows libclang API users to opt into storing PCH in memory
instead of temporary files. The option can be set only during CXIndex
construction to avoid multithreading issues and confusion or bugs if
some preambles are stored in temporary files and others - in memory.

The added API works as expected in KDevelop:
https://invent.kde.org/kdevelop/kdevelop/-/merge_requests/283

Differential Revision: https://reviews.llvm.org/D145974