[clangd] Rename global-symbol-builder to clangd-indexer.

Summary:
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

llvm-svn: 342123