[clangd] Ignore more implicit nodes in computing selection.

Summary:
The DeclRefExpr for the callee of overloaded `operator()` and `operator[]` are
assigned the range of the paren/bracket lists in the AST.
These are better thought of as implicit (at least `()` - `[] is murkier).
But there's no bit on Expr for implicit, so just ignore them on our side.

While here, deal with the case where an implicit stmt (e.g. implicit-this)
is wrapped in an implicit cast. Previously we ignored the statement but not
the cast, and so the cast ended up being selected.

Fixes https://github.com/clangd/clangd/issues/195

Reviewers: kadircet, lh123

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70194