Change pretty printing of constant so that the attributes precede the value.

This does create an inconsistency between the print formats (e.g., attributes are normally before operands) but fixes an invalid parsing & keeps constant uniform wrt itself (function or int attributes have type at same place). And specifying the specific type for a int/float attribute might get revised shortly.

Also add test to verify that output printed can be parsed again.

PiperOrigin-RevId: 221923893