[clangd] Support textEdit in addition to insertText.

Summary:
Completion replies contains textEdits as well. Note that this change
relies on https://reviews.llvm.org/D50443.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: mgrang, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50449

llvm-svn: 339543