[ASTGen] Generate MagicIdentifierLiteralExpr

[rintaro]

E.g. #file. These are parsed as MacroExpansionExprSyntax in SwiftParser.

Also while I'm here, eliminate SYNTAX_KIND from MagicIdentifierKinds.def because nothing is using it anymore.

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test

[hamishknight]

Also need to handle things like #error, #warning, #assert, #fileLiteral, #colorLiteral, and #imageLiteral, right? Would it be worth bridging an enum for POUND_KEYWORD such that we make sure we don't miss any cases here?

[rintaro]

Huh, I thought #fileLiteral et al were removed from the language, yeah we might want a bridging enum for POUND_OBJECT_LITERAL (i.e. ObjectLiteralExpr::LiteralKind).

As for #error, #warning, and #assert, they are not expressions so we can't handle it here.
#assert is a statement, #warrning and #error are decl, considering how to handle them in ASTGen and POUND_KEYWORD would include some noise (i.e. #if, #sourceLocation etc.), I'm not sure POUND_KEYWORD bridging enum would be beneficial. I will think about it.

Anyway, thank you for reminding me!

[hamishknight]

As for #error, #warning, and #assert, they are not expressions so we can't handle it here.

We'd want to diagnose them though, right? AFAIK swift-syntax won't reject them