swift234

Implement <rdar://problem/16204675> Need #elseif

This restructures IfConfigDecl/Stmt to be a list of clauses controlled
by a condition.  This makes it straight-forward to drop in #elseif support.

While I'm in here, this patch moves checking for extraneous stuff at the
end of the #if line from the lexer to the parser.  This means that you can
now put a comment on the same line as a #if/#else/#elseif/#endif.



Swift SVN r16912