fix PR4073 by making designated initializer checking code use

VerifyIntegerConstantExpression instead of isIntegerConstantExpr.
This makes it ext-warn but tolerate things that fold to a constant
but that are not valid i-c-e's.

There must be a bug in the i-c-e computation though, because it
doesn't catch this case even with pedantic.

This also switches the later code to use EvaluateAsInt which is
simpler and handles everything that evaluate does.

llvm-svn: 70081