[clangd] Return clangd::TextEdit in ClangdServer::rename. NFC

Summary:
Instead of tooling::Replacement. To avoid the need to have contents of
the file at the caller site. This also aligns better with other methods
in ClangdServer, majority of those already return LSP-specific data
types.

Reviewers: hokein, ioeric, sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60179

llvm-svn: 357561