## Why?
The explanation of x and y is reversed.

https://github.com/ruby/ruby/blob/ddbd64400199fd408d23c85f9fb0d7f742ecf9e1/re.c#L251-L256
```
long
rb_memsearch(const void *x0, long m, const void *y0, long n, rb_encoding *enc)
{
    const unsigned char *x = x0, *y = y0;

    if (m > n) return -1;
```

```
../../.././include/ruby/internal/special_consts.h:349:36: error: conversion to ‘VALUE’ {aka ‘long unsigned int’} from ‘int’ may change the sign of the result [-Werror=sign-conversion]
  349 |     return RB_SPECIAL_CONST_P(obj) * RUBY_Qtrue;
      |            ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
```

[Bug #20650]

The capture group allocates memory that is leaked when it times out.

For example:

    re = Regexp.new("^#{"(a*)" * 10_000}x$", timeout: 0.000001)
    str = "a" * 1000000 + "x"

    10.times do
      100.times do
        re =~ str
      rescue Regexp::TimeoutError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    34688
    56416
    78288
    100368
    120784
    140704
    161904
    183568
    204320
    224800

After:

    16288
    16288
    16880
    16896
    16912
    16928
    16944
    17184
    17184
    17200

Since 730e3b2ce01915c4a98b79bb281b2c38a9ff1131
("Stop exposing `rb_str_chilled_p`"), we noticed a speed loss on a few
benchmarks that are string operations heavy. This is partially due to
routines no longer having the options to inline rb_check_frozen_inline()
in non-LTO builds. Make it an inlining candidate again to recover speed.

Testing this patch on my machine, the fannkuchredux benchmark gets a
1.15 speed-up with YJIT and 1.03 without YJIT.

And move it back to a public header because Doxygen might not be
scanning the .c files.

[Feature #18776]

[Feature #20589]

This feature is no longer possible under current design; now that our GC
is pluggable, we cannot assume what was achieved by this compiler flag
is always possble by the dynamically-loaded GC implementation.