Instead of the simple "expected identifier in declaration", the error will now read "keyword '%' cannot be used as an identifier here", and will be accompanied by a note suggesting escaping the keyword with backticks, as well as a fixit.
https://bugs.swift.org/browse/SR-3167
Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK. The driver was defaulting to the
host OS. Thus, we could not run the tests when the standard library was
not built for OS X.
Swift SVN r24504
Lex a backtick-enclosed `[:identifier_start:][:identifier_cont:]+` as an identifier, even if it's a Swift keyword. For now, require that the escaped name still be a valid identifier, keyword collisions notwithstanding. (We could in theory allow an arbitrary string, but we'd have to invent a mangling for non-identifier characters and do other tooling which doesn't seem productive.)
Swift SVN r14671
