ConcatAnalyzer improvements and non-falsy-string fixes.

[AndrolGenhald]

Deduplicate code.
Improve type inference.
Allow literal type inference when only one side has multiple types (fixes #5483).
Fix invalid type inference with negative int as right operand.

[AndrolGenhald]

I'm tempted to offload most of the analysis here to CastAnalyzer, and since PHP (probably?) handles concatenation by casting everything to string first InvalidCast might be more applicable than InvalidOperand anyway. The only downside I see is that users would have to update their suppressions.

@muglug @weirdan thoughts?

[AndrolGenhald]

Heads up @orklah, since you added the int concatenation to numeric string inference you may want to be aware that this now requires the right operand to be a non-negative int.

[orklah]

Heads up @orklah, since you added the int concatenation to numeric string inference you may want to be aware that this now requires the right operand to be a non-negative int.

oh nice, didn't thought of that one at the time

[AndrolGenhald]

I stumbled on some issues with non-falsy-string while fixing newly detected issues in the codebase, so I fixed those as well.

[muglug]

Thanks! This is a great addition/refactor!

I wonder whether it might be better in the long run to make, "empty", "falsy" and "lowercase" attributes of a string type, with rules about which attributes can intersect. That might make the logic more complex though, not less.