Add initializer to `LabeledExprSyntax` that automatically adds a colon if the label exists

[ahoppen]

Currently, if you do the following

let node = EnumCaseParameterSyntax(firstName: "label", type: TypeSyntax("MyType"))
print(node.formatted())

you get label MyType, which is surprising because the colon is missing and you would expect the colon to be present if there is a first name of the enum case parameter.

To fix this, we should add a hand-written initializer of EnumCaseParameterSyntax, which takes a non-optional firstName and automatically adds a colon, because it knows that the firstName exists. The above call would pick that overload and thus add a colon.

And the same strategy can also be applied

• to other nodes that have an optional colon
• ClosureCaptureSyntax.equal
• to automatically add parentheses for the following nodes if arguments are present
  • AttributeSyntax
  • MacroExpansionDeclSyntax
  • ClosureCaptureModifierSyntax
  • FunctionCallExprSyntax
  • FreestandingMacroExpansionSyntax

rdar://107794232