swift234

Transform EditorPlaceholderExpr into trap if executed in playground mode.

Allow untyped placeholder to take arbitrary type, but default to Void.
Add _undefined<T>() function, which is like fatalError() but has
arbitrary return type. In playground mode, merely warn about outstanding
placeholders instead of erroring out, and transform placeholders into
calls to _undefined(). This way, code with outstanding placeholders will
only crash when it attempts to evaluate such placeholders.

<rdar://problem/21167372> transform EditorPlaceholderExpr into fatalError()

Swift SVN r31481