[Attributes] Support int attributes with zero value

This regularly comes up as a stumbling stone when adding int
attributes: They currently need to be encoded in a way to avoids
the zero value.

This adds support for zero-value int attributes by a) making the
ctor determine int/enum attribute based on attribute kind, not
whether the value is non-zero and b) switching getRawIntAttr()
to return an Optional, so that it's possible to distinguish a zero
value from non-existence.

Differential Revision: https://reviews.llvm.org/D135572