Improve -Wconstant-conversion

Switch the evaluation from isIntegerConstantExpr to EvaluateAsInt.
EvaluateAsInt will evaluate more types of expressions than
isIntegerConstantExpr.

Move one case from -Wsign-conversion to -Wconstant-conversion.  The case is:
1) Source and target types are signed
2) Source type is wider than the target type
3) The source constant value is positive
4) The conversion will store the value as negative in the target.

llvm-svn: 259271