[ondrejmirtes]

Stacked on #6937. Review only the last commit.

Adds Type::getPossiblyLooselyEqualType(PhpVersion) and Type::getAlwaysLooselyEqualType(PhpVersion). For the configured PHP version, the first is the set of values $this == $x can be true for, and the second is the set it is always true for. LooseComparisonHelper computes both.

How they are used

• == narrowing. The true branch intersects each side with the other side's possible set. The false branch removes the other side's always set. This replaces the == 0 and == '' special cases that hedged between PHP 7 and PHP 8.
• resolveEqualType(). When looseCompare() is undecided, it now answers false when the other side is outside the possible set, and true when it is inside the always set.
• Loose in_array(). It now narrows the needle too, and makes the haystack non-empty in the true branch.
  • An enum is only compared strictly when the other side cannot be true.
  • Two strings are only compared strictly when they cannot both be numeric.
• Cast truthiness. (int), (float) and (string) get their own zero and falsy sets, because (int) 'abc', (int) 0.5 and (string) 0 do not follow == 0 and == ''.

mixed stays mixed. When the true branch would turn mixed into a union containing objects such as GMP|Stringable, the subject is left as mixed. This covers explicit, implicit and subtracted mixed, and templates bounded by mixed, for ==, switch and loose in_array(). Enum cases are precise enough, so they still narrow. Every other type still narrows, for example string|bool|null == 'a' gives 'a'|true and object|string == 'a' gives 'a'|GMP|Stringable. Note that mixed == 0 and mixed == '' now stay mixed.

PHP version ranges. Type methods still take one PhpVersion. Callers that have a scope expand Scope::getPhpVersion() with LooseComparisonHelper::getRepresentativePhpVersions(): one version per range that compares alike, split at 8.0 and 8.4. The possible sets are unioned, the always sets are intersected, and looseCompare() is constant only when every version agrees. With a 7.4–8.5 range, 0 == 'abc' is bool and $int == 'abc' narrows to 0. Inside if (PHP_VERSION_ID >= 80000) the PHP 8 rules apply. Scope-less initializer expressions keep the single configured version.

Runtime facts in the sets. These were measured on PHP 7.4, 8.0 and 8.5:

• Every object equals 1 and true.
• On PHP 8, only Stringable and GMP objects equal strings. On PHP 7, any object can.
• Only GMP, BcMath\Number and SimpleXMLElement objects equal other numbers or false.
• NAN equals only true. INF == 'INF' is true from PHP 8.

Trade-off. Two numeric strings compare as numbers ('1E3' == '1e3'), so a pair of numeric strings is no longer narrowed as if it were ===. Some types get noisier but sound unions as a result, for example in bug-12312.

The Turbo mirrors are in the same commit. Self-analysis is byte-identical with Turbo on and off.

Closes phpstan/phpstan#8224