[clangd] Rename global-symbol-builder to clangd-indexer.
Given that the indexer binary is put directly into ./bin directory
when built, 'clangd-' prefix seems to provide better context to the
reader than 'global-'.
The new name is also shorter and easier to type.
Reviewers: ioeric, sammccall, kadircet
Reviewed By: ioeric, sammccall
Subscribers: kbobyrev, ilya-biryukov, mgorny, MaskRay, jkorous, arphaman, cfe-commits
Differential Revision: https://reviews.llvm.org/D51987