Fill out unimplemented expression cases in the ASTPrinter

In #40691 we added initial support for printing Expr nodes in the ASTPrinter, and #64667 filled out most of the rest. However there are still some expressions left unimplemented, including:

• All the ImplicitConversionExpr subclasses, we should stamp out their definitions as printing their sub-expression, using a macro
• InterpolatedStringLiteralExpr (this is a tricky case since we'd need to reconstruct the string literal from the TapExpr, or otherwise preserve its original form)

These are partially implemented, with outstanding FIXMEs:

• ClosureExpr
• CaptureListExpr
• KeyPathExpr

Some extras tests would be good for:

• SubscriptExpr (would be great to have a test exercising some other call patterns, e.g multiple arguments, argument labels)
• TupleElementExpr
• ObjCSelectorExpr
• DynamicSubscriptExpr
• EnumIsCaseExpr
• MakeTemporarilyEscapableExpr
• EditorPlaceholderExpr
• DotSyntaxBaseIgnoredExpr