swift234

Several minor changes:
 - Strength reduce isAtStartOfBindingName() to just check for 
   identifier or _ and inline into its two callers.
 - Rename Token::isIdentifierOrNone to isIdentifierOrUnderscore.
 - Teach InVarOrLetPattern about matching patterns, so that the
   parser knows when it is parsing an expression as a matching
   pattern but is not yet inside a let/var pattern.
 - Use newfound knowledge of matching patterns to refine handling
   of unexpected let/var when parsing an expression, but not in a
   pattern context, slightly improving QoI in invalid cases.



Swift SVN r26172