new instances of -Wunused-variable w/ clang-18

[nickdesaulniers]

@nathanchance reported that some CI jobs failed overnight with new instances of -Wunused-variable.
example: https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/6033685211/job/16371310061#step:6:101

He also tracked it down to
llvm/llvm-project@92023b1

There's discussion on
https://reviews.llvm.org/D152495
about splitting this out into it's own warning.
https://reviews.llvm.org/D152495#4628887

Here's a full trace from mainline (KCFLAGS=-fmacro-backtrace-limit=0):

In file included from arch/x86/kernel/asm-offsets.c:9:
In file included from ./include/linux/crypto.h:17:
In file included from ./include/linux/slab.h:16:
In file included from ./include/linux/gfp.h:7:
In file included from ./include/linux/mmzone.h:22:
In file included from ./include/linux/mm_types.h:11:
In file included from ./include/linux/rbtree.h:24:
./include/linux/rcupdate.h:864:2: error: unused variable '__UNIQUE_ID___addressable___SCK__preempt_schedule89' [-Werror,-Wunused-variable]
  864 |         preempt_enable();
      |         ^~~~~~~~~~~~~~~~
./include/linux/preempt.h:223:3: note: expanded from macro 'preempt_enable'
  223 |                 __preempt_schedule(); \
      |                 ^~~~~~~~~~~~~~~~~~~~
./arch/x86/include/asm/preempt.h:127:2: note: expanded from macro '__preempt_schedule'
  127 |         __STATIC_CALL_MOD_ADDRESSABLE(preempt_schedule); \
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/static_call_types.h:86:45: note: expanded from macro '__STATIC_CALL_MOD_ADDRESSABLE'
   86 | #define __STATIC_CALL_MOD_ADDRESSABLE(name) __STATIC_CALL_ADDRESSABLE(name)
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/static_call_types.h:53:2: note: expanded from macro '__STATIC_CALL_ADDRESSABLE'
   53 |         __ADDRESSABLE(STATIC_CALL_KEY(name))
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:214:2: note: expanded from macro '__ADDRESSABLE'
  214 |         ___ADDRESSABLE(sym, __section(".discard.addressable"))
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:212:3: note: expanded from macro '___ADDRESSABLE'
  212 |                 __UNIQUE_ID(__PASTE(__addressable_,sym)) = (void *)&sym;
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:180:29: note: expanded from macro '__UNIQUE_ID'
  180 | #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
././include/linux/compiler_types.h:75:22: note: expanded from macro '__PASTE'
   75 | #define __PASTE(a,b) ___PASTE(a,b)
      |                      ^~~~~~~~~~~~~
././include/linux/compiler_types.h:74:23: note: expanded from macro '___PASTE'
   74 | #define ___PASTE(a,b) a##b
      |                       ^~~~
<scratch space>:115:1: note: expanded from here
  115 | __UNIQUE_ID___addressable___SCK__preempt_schedule89
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[nickdesaulniers]

clang bug: https://reviews.llvm.org/D152495#4631829

[nickdesaulniers]

reverted: llvm/llvm-project@f0f395e

[nickdesaulniers]

Adding some tests so this doesn't regress again: https://reviews.llvm.org/D159307.