Infer literal types for word date format characters - #6838

The a, A, S, D, l, M, and F format characters of date(), date_format(), and DateTimeInterface::format() now produce unions of their possible values, alongside the numeric single-character formats added in #721. PHP formats these characters from hard-coded English tables in ext/date/php_date.c, so the values do not depend on the locale.

Removal of the non-empty-string widening

Adding the cases alone is not enough: getTypeFromFormatType() ended with

if ($type->isNumericString()->no() && $formatType->isNonEmptyString()->yes()) {
	$type = TypeCombinator::union($type, new IntersectionType([
		new StringType(), new AccessoryNonEmptyStringType(),
	]));
}

which widens any non-numeric constant result to non-empty-string, so the new unions came out as non-empty-string. The same happens today for formats without format characters, e.g. date(' ') is non-empty-string rather than ' '.

This block was added in #2888 for phpstan/phpstan#10468 (a non-empty format should give a non-empty result). That guarantee does not depend on it:

• A non-constant format is handled by the count($types) === 0 branch, which already returns non-empty-string for a non-empty-string format.
• For a constant non-empty format, every type buildReturnTypeFromFormat() returns is already non-empty: the explicit cases, ConstantStringType($date), numeric-string, and non-falsy-string.

So the block is removed rather than worked around. The bug-10468.php and bug-6613.php tests still pass.

Tests

• nsrt/bug-15537.php is the reproducer from the issue. It fails before this change (non-falsy-string for every assertion) and passes after.
• nsrt/date-format.php expected non-falsy-string for D; it now expects the weekday union.

Closes phpstan/phpstan#15537

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