swift234

Support escape sequences in @available() messages.

Allow API providers to use escaped characters in the messages of @available()
attributes. For example, for a reference to a declaration annotated with:

@available(*, unavailable, message="use print(\"\")")
public func println() {
...
}

the message displayed to the user will be:

'println() is unavailable: use print("")'.

This also supports unicode escapes (e.g. \u{1F43C}).

A complication is that AvailableAttrs imported from clang already have their
messages unescaped, so this patch keeps track of whether an availability
attribute came from clang and, if so, does not unescape the message again when
emitting the diagnostic. We could avoid this extra book-keeping by immediately
unescaping Swift messages during parsing, keeping them in an unescaped
canonical form, and then escaping them in ASTPrinter when we print the
attributes. rdar://problem/20985357 tracks keeping AvailableAttrs messages in
a canonical form.

rdar://problem/20789407

Swift SVN r28658