[clangd] Support hover on __func__ etc (PredefinedExpr)

Expose these as variables as that's what the standard calls them (and D131175).

To make this work, we also fix a bug in SelectionTree: PredefinedExpr has
an implicit/invisible StringLiteral, and SelectionTree should not traverse
implicit things.

Reviewed By: ckandeler

Differential Revision: https://reviews.llvm.org/D132135