swift234

Start parsing names with argument labels.

Basic implementatation of SE-0021, naming functions with argument
labels. Handle parsing of compound function names in various
unqualified-identifier productions, updating the AST representation of
various expressions from Identifiers to DeclNames. The result doesn't
capture all of the source locations we want; more on that later.

As part of this, remove the parsing code for the "selector-style"
method names, since we now have a replacement. The feature was never
publicized and doesn't make sense in Swift, so zap it outright.