Improve qualifier formatting outputted for InferenceVisitor commonAssignmentCheck

[jyluo]

If the ATM passed to commonAssignmentCheck is the varAnnots representing constant slots, and there is a failure of the subtype check, the error message is not helpful:

INFO: ByteCode.java:14: error: [argument.type.incompatible] incompatible types in argument.
        StringBuilder sb2 = new StringBuilder(UnitsTools.s);
                                                        ^
  found   : @VarAnnot(14) int
  required: @VarAnnot(10) int

Here the example is for units, of which the real annotation is @VarAnnot(14) = @s and @VarAnnot(10) = @Dimensionless.

[jyluo]

Likely code that needs to be changed:
https://github.com/opprop/checker-framework-inference/blob/master/src/checkers/inference/InferenceVisitor.java#L486