[AST] RecursiveASTVisitor: Don't traverse the alias deduction guides in the default mode. (#91454)

By default (`shouldVisitImplicitCode()` returns `false`), RAV should not
traverse AST nodes that are not spelled in the source code. Deduction
guides for alias templates are always synthesized, so they should not be
traversed.

This is usually done by checking the implicit bit of the Decl. However,
this doesn't work deduction guides that are synthesized from explicit
user-defined deduction guides, as we must maintain the explicit bit to
ensure correct overload resolution.