1. Array type parsing for postfix array types Int[]. We now handle this
in the parser, but remove the AST representation of this old form. We
also stop making vague promises about the future by saying that "fixed
size arrays aren't supported... yet". Removal of this fixes a compiler
crasher too.
2. Remove the special case support for migrating @autoclosure from types
to parameters, which was Swift 1.0/1.1 syntax. The world has moved or
we don't care anymore.
3. Remove upgrade support for # arguments (nee "backtick" arguments), which
was a Swift 1.x'ism abolished in an effort to simplify method naming
rules.
NFC on valid code.
