In order to get rid of the flag in `BuilderHelpers::normalizeNameCommon()` I have moved all the logic related to the normalization of the name to the `BuilderHelpers::normalizeName()` method and expr-related stuff to the `BuilderHelpers::normalizeNameOrExpr()` method which later calls the basic `normalizeName()` as well

Fixes issue #800.

With the upcoming addition of intersection types, a type can
be Identifier|Name|NullableType|UnionType|IntersectionType, which
is quite the mouthful. Give NullableType and UnionType a common
base class ComplexType, which does not have any behavior, but
allows to write these types (and check them in instanceof) more
easily.

This already works because we don't validate initializer contents,
everything is accepted from a parser perspective.

I'm somewhat unsure about the AST structure here.
VariadicPlaceholder is not a general expression. Maybe Arg->expr
should be Expr|VariadicPlaceholder? Or possibly the call arguments
should be an array of Arg|VariadicPlaceholder?

Print comma before rather than after comments. Also switch to multiline mode if inserting an item with comments.

Fixes #804.

And don't store it in an Arg.

This provides a helper to determine whether a call is a first-class
callable, and a way to strip the args type to Arg[] if it isn't.