Remove an extraneous QualType from CastExpr, it's type is always

the result type of the expr node.

Implement isIntegerConstantExpr for ImplicitCastExpr nodes the same
was as for CastExpr nodes.

Implement proper sign/zero extension as well as truncation and noop
conversion in the i-c-e evaluator.  This allows us to correctly
handle i-c-e's like these:

char array[1024/(sizeof (long))];
int x['\xBb' == (char) 187 ? 1: -1];

this implements test/Sema/i-c-e2.c

llvm-svn: 39888