- Remove unused STRING token from lexer & parser

  - Changed parser to always use parenthesis on ConstExprs to be consistent
  - Parser now passes TRUE and FALSE tokens as a special case of the ConstExpr
    machinery instead of a special case of constant int stuff
  - Fix the AsmParser to use ValueRef ::= ConstExpr, and remove
    ResolvedVal ::= ConstExpr this allows constexprs to be used in PHI nodes

llvm-svn: 3362