gen_stub: Fix handling of escape sequences in generated C strings

[NattyNarwhal]

When handling sequences like this in a stub:

<?php
class Whatever {
    public static string $foobar1 = "CCC \n\r\t\v\e\f\\\$\"\101\x41\u{41} CCC";
    public static string $foobar2 = 'CCC \n\r\t\v\e\f\\\$\"\101\x41\u{41} CCC';
}

...properly generate C headers that properly escape the string. Otherwise, the differing escaping rules and differences between PHP's single and double quoted strings could lead to mangled headers.

The output of these strings after the stub has been generated:

string(20) "CCC

        $"AAA CCC"
string(41) "CCC \n\r\t\v\e\f\\\\$\"\101\x41\u{41} CCC"

And the generated arginfo:

        zval property_foobar1_default_value;
        zend_string *property_foobar1_default_value_str = zend_string_init("CCC \n\r\t\v\x1b\f\\\$\"AAA CCC", strlen("CCC \n\r\t\v\x1b\f\\\$\"AAA CCC"), 1);
        ZVAL_STR(&property_foobar1_default_value, property_foobar1_default_value_str);
        zend_string *property_foobar1_name = zend_string_init("foobar1", sizeof("foobar1") - 1, true);
        zend_declare_typed_property(class_entry, property_foobar1_name, &property_foobar1_default_value, ZEND_ACC_PUBLIC|ZEND_ACC_STATIC, NULL, (zend_type) ZEND_TYPE_INIT_MASK(MAY_BE_STRING));
        zend_string_release_ex(property_foobar1_name, true);

        zval property_foobar2_default_value;
        zend_string *property_foobar2_default_value_str = zend_string_init("CCC \\n\\r\\t\\v\\e\\f\\\\\\\\$\\\"\\101\\x41\\u{41} CCC", strlen("CCC \\n\\r\\t\\v\\e\\f\\\\\\\\$\\\"\\101\\x41\\u{41} CCC"), 1);
        ZVAL_STR(&property_foobar2_default_value, property_foobar2_default_value_str);
        zend_string *property_foobar2_name = zend_string_init("foobar2", sizeof("foobar2") - 1, true);
        zend_declare_typed_property(class_entry, property_foobar2_name, &property_foobar2_default_value, ZEND_ACC_PUBLIC|ZEND_ACC_STATIC, NULL, (zend_type) ZEND_TYPE_INIT_MASK(MAY_BE_STRING));
        zend_string_release_ex(property_foobar2_name, true);

Note that the PHP escape sequence "$" will generate a warning with clang at least, as C doesn't have an equivalent:

/Users/calvin/src/php-src/ext/zend_test/test_arginfo.h:799:90: warning: unknown escape sequence '\$' [-Wunknown-escape-sequence]
[...]

I'm also unsure how best to make tests for gen_stub, so I haven't included any beyond mentioning how to test manually with this commit.

Fixes GH-22169.

[TimWolla]

I'm also unsure how best to make tests for gen_stub, so I haven't included any beyond mentioning how to test manually with this commit.

Add it to the ext/zend_test stub.