Allow Builder to create function-type constants

A recent change made ConstantOp::build accept a NumericAttr or assert that a
generic Attribute is in fact a NumericAttr.  The rationale behind the change
was that NumericAttrs have a type that can be used as the result type of the
constant operation.  FunctionAttr also has a type, and it is valid to construct
function-typed constants as exercised by the parser.mlir test.  Relax
ConstantOp::build back to take a generic Attribute.  In the overload that only
takes an attribute, assert that the Attribute is either a NumericAttr or a
FunctionAttr, because it is necessary to extract the type.  In the overload
that takes both type type and the attribute, delegate the attribute type
checking to ConstantOp::verify to prevent non-Builder-based Op construction
mechanisms from creating invalid IR.
PiperOrigin-RevId: 234798569