[clangd] expose MergedIndex class

Summary:
This allows inheriting from it, so index() can ga away and allowing
TestTU::index) to be fixed.

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52250

llvm-svn: 343780