REVIEW fix(SniperPrettyPrinter): fix pretty-printing of joint declarations (#3386)

[nharrand]

Reproduces #3386

[monperrus]

Ohh that will be tough

the ElementSourceFragment is int a;|99, 108|int a, c;|, it covers two elements while being attached to only int a (elementFragment.getElement() == <int a> only)

[monperrus]

The ElementSourceFragment of int c is merged into that of int a because it is a strict subset.

[monperrus]

Here is a first fix so that @fermadeiral can proceed.

It's a partial fix, fields with joint declarations won't be snipered, but the improvement is that the resulting code will compile.

It's mergeble but it does not close #3386.

[fermadeiral]

Thanks @monperrus!