[wgsl-out] Correct handling of named pointer expressions.

Treat expressions in `Function::named_expressions` like WGSL `let` declarations,
assuming that the Load Rule was applied to the rhs of the declaration, meaning
that their values are always `Indirection::Ordinary`.

Split `write_expr_plain_form` out from `write_expr_with_indirection`, to clean
up the parenthesis generation: no more `opened_paren` variable, just function
calls. This makes the early return for named expressions neater.

Fixes #1382.