Fix a cluster of related issues involving value-dependence and constant

expression evaluation:
 - When folding a non-value-dependent expression, we may try to use the
   initializer of a value-dependent variable. If that happens, give up.
 - In C++98, actually check that a const, non-volatile DeclRefExpr inside an ICE
   is of integral or enumeration type (a reference isn't OK!)
 - In C++11, DeclRefExprs for objects of const literal type initialized with
   value-dependent expressions are themselves value-dependent.
 - So are references initialized with value-dependent expressions (though this
   case is missing from the C++11 standard, along with many others).

llvm-svn: 144056