Add a dynamic return type extension for clamp()

[Amoifr]

Closes phpstan/phpstan#15366

PHP 8.6 adds clamp(mixed $value, mixed $min, mixed $max): mixed. php_math_clamp() returns a copy of one of its three arguments: $max when $max < $value, otherwise $min when $value < $min, otherwise $value.

The new extension rewrites the call into that ternary, with AlwaysRememberedExpr, the way MinMaxFunctionReturnTypeExtension handles min($a, $b) and max($a, $b). So clamp($int, 1, 10) is int<1, 10>, clamp($int, 1.0, 10.0) is 1.0|10.0|int<1, 10>, and constant arguments give a constant result. Arrays, which PHP compares by size first, and unpacked arguments keep the native mixed.

The type inference test needs clamp() at runtime, so it is gated with // lint >= 8.6.