Add equals() and hashCode() to UninterpretedFunctionDeclaration
(a function can be declared various times and refers to the same function,
so missing equals() could lead to confusing behavior).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@15108 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c