[PATCH] Remove rbimpl_rstring_getmem() usage as workaround for GCC 15.2.1 optimization bug. [Bug #21655]

	[PATCH] include/ruby/internal/core/rstring.h: Remove rbimpl_rstring_getmem() definition.

	[PATCH] simplify RSRING_GETMEM() definition.

	[PATCH] [Bug #21629] Initialize `struct RString`

which appears to be missed in the previous commit for some reason.

* Use FL_USER0 for ELTS_SHARED

This makes space in RString for two bits for chilled strings.

* Mark strings returned by `Symbol#to_s` as chilled

[Feature #20350]

`STR_CHILLED` now spans on two user flags. If one bit is set it
marks a chilled string literal, if it's the other it marks a
`Symbol#to_s` chilled string.

Since it's not possible, and doesn't make much sense to include
debug info when `--debug-frozen-string-literal` is set, we can't
include allocation source, but we can safely include the symbol
name in the warning message, making it much easier to find the source
of the issue.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

---------

Co-authored-by: Étienne Barrié <etienne.barrie@gmail.com>
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

[Feature #19998]

Fixes:

```
/usr/local/ruby/include/ruby-3.3.0+0/ruby/internal/core/rtypeddata.h:467:33:
error: invalid conversion from ‘void*’ to ‘parser_t*’ [-fpermissive]
  467 |     (sval) = RTYPEDDATA_GET_DATA(result); \
      |              ~~~~~~~~~~~~~~~~~~~^~~~~~~~
      |                                 |
      |                                 void*
```

This commit adds a new flag RUBY_TYPED_EMBEDDABLE that allows the data
of a TypedData object to be embedded after the object itself. This will
improve cache locality and allow us to save the 8 byte data pointer.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>

Since edf01d4e82d8e44ee30ec41fbcb7f802bc8b8c5d, fake string treats
NULL as an empty string.