[clangd] Fix rename for symbol introduced by UsingDecl

Fixes https://github.com/clangd/clangd/issues/170

This patch actually consists of 2 fixes:

1) Add handling for UsingShadowDecl to canonicalRenameDecl().
   This fixes the issue described in https://github.com/clangd/clangd/issues/170.

2) Avoid the "there are multiple symbols under the cursor error" by applying similar
   logic as in https://reviews.llvm.org/D133664.
   This also partly fixes https://github.com/clangd/clangd/issues/586.

Differential Revision: https://reviews.llvm.org/D135489