[clangd] Remove FilterText from the index.

Summary:
It's almost always identical to Name, and in fact we never used it (we used name
instead).
The only case where they differ is objc method selectors (foo: vs foo:bar:).
We can live with the latter for both name and filterText, so I've made that
change too.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D48375

llvm-svn: 335321