Building certain ruby gem native extensions (such as thrift), with clang
12.0.0 or later fails, because they have -Werror in their CFLAGS,
resulting in complaints about the expansion of the `rb_intern()` macro:

```
current directory: /wrkdirs/usr/ports/devel/rubygem-thrift/work/stage/usr/local/lib/ruby/gems/2.7/gems/thrift-0.14.0/ext
make "DESTDIR="
compiling binary_protocol_accelerated.c
binary_protocol_accelerated.c:404:68: error: '(' and '{' tokens introducing statement expression appear in different macro expansion contexts [-Werror,-Wcompound-token-split-by-macro]
  VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1847:23: note: expanded from macro 'rb_intern'
        __extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
                      ^
binary_protocol_accelerated.c:404:68: note: '{' token is here
  VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1847:24: note: expanded from macro 'rb_intern'
        __extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1832:5: note: expanded from macro 'RUBY_CONST_ID_CACHE'
    {                                                   \
    ^
```

Part of the `rb_intern()` macro expands to `(RUBY_CONST_ID_CACHE((ID),
(str)))`, and in turn `RUBY_CONST_ID_CACHE()` expands to a brace
enclosed compound statement. The intended effect is to get a gcc
statement expression, which is normally delimited by `({ ... })`.

However, clang 12.0.0 and later have a warning enabled by default, about
pasting together the `(` and `{` tokens via different macros (see
<https://github.com/llvm/llvm-project/commit/0e00a95b4fad5e72851de012d3a0b2c2d01f8685>).

To work around this warning:
* Add `RUBY_CONST_ID_CACHE_NB()` (i.e. no-brace) which contains the code
  itself, without any braces
* `RUBY_CONST_ID_CACHE()` which uses `RUBY_CONST_ID_CACHE_NB()`, but
   puts braces around it (so no existing code using this macro breaks)
* Finally, change `rb_intern()` so the `__extension__` directly creates
  a gcc statement expression, using the `RUBY_CONST_ID_CACHE_NB()` macro

This reverts commit a6b50f379d4b71acac73ac6b22cbe2d2c5b50b3f.

(old)
test.rb:4: warning: The last argument is used as the keyword parameter
test.rb:1: warning: for `foo' defined here; maybe ** should be added to the call?

(new)
test.rb:4: warning: The last argument is used as keyword parameters; maybe ** should be added to the call
test.rb:1: warning: The called method `foo' is defined here

This removes the security features added by $SAFE = 1, and warns for access
or modification of $SAFE from Ruby-level, as well as warning when calling
all public C functions related to $SAFE.

This modifies some internal functions that took a safe level argument
to no longer take the argument.

rb_require_safe now warns, rb_require_string has been added as a
version that takes a VALUE and does not warn.

One public C function that still takes a safe level argument and that
this doesn't warn for is rb_eval_cmd.  We may want to consider
adding an alternative method that does not take a safe level argument,
and warn for rb_eval_cmd.

Limit strict function signature check with RUBY_METHOD_FUNC in C++
to bundled libraries only.  [Bug #16271]

This macro is used here before defined in ruby.h.

There is no need for these in Ruby 3.0, and the plan is to
remove them.

This adds rb_funcall_passing_block_kw, rb_funcallv_public_kw,
and rb_yield_splat_kw.  This functions are necessary to easily
handle cases where rb_funcall_passing_block, rb_funcallv_public,
and rb_yield_splat are currently used and a keyword argument
separation warning is raised.