Fix releaseClang segfault: declare invoke_exit_action [[noreturn]], remove __builtin_unreachable() from UNREACHABLE() (#10295)

Commit d46fbad3 added `__builtin_unreachable()` to the `UNREACHABLE()`
macro in Clang builds to suppress `-Wimplicit-fallthrough` warnings. In
Release+Clang mode, this causes UB: Clang can legally eliminate the
`invoke_exit_action` call (since reaching `__builtin_unreachable()` is
UB, the compiler assumes the entire branch is dead), so when
`UNREACHABLE()` is actually hit, the function continues with
uninitialized state — producing the segfault observed in the FPA C
example.

## Changes

- **`src/util/debug.h`** — Declare `invoke_exit_action` as
`[[noreturn]]`. This is semantically accurate: the function always
terminates via `exit()` or `throw`, never returning normally. The
`[[noreturn]]` annotation naturally suppresses `-Wimplicit-fallthrough`
after `UNREACHABLE()` without any UB.
- **`src/util/debug.h`** — Remove `__compiler_unreachable` macro and its
use in `UNREACHABLE()`. With `[[noreturn]]` on `invoke_exit_action`, it
is redundant.
- **`src/util/debug.cpp`** — Add `[[noreturn]]` to the
`invoke_exit_action` definition to match.

```cpp
// Before (broken in Release+Clang: UB allows optimizer to eliminate invoke_exit_action)
# define UNREACHABLE() { notify_assertion_violation(...); invoke_exit_action(ERR_UNREACHABLE); }; __builtin_unreachable()

// After (invoke_exit_action is [[noreturn]], no UB)
# define UNREACHABLE() { notify_assertion_violation(...); invoke_exit_action(ERR_UNREACHABLE); } ((void) 0)
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/util/debug.cpp

@@ -122,7 +122,7 @@ void set_default_exit_action(exit_action a) {
     g_default_exit_action = a;
 }
 
-void invoke_exit_action(unsigned int code) {
+[[noreturn]] void invoke_exit_action(unsigned int code) {
     exit_action a = get_default_exit_action();
     switch (a) {
     case exit_action::exit:

src/util/debug.h

@@ -42,7 +42,7 @@ enum class exit_action {
 };
 exit_action get_default_exit_action();
 void set_default_exit_action(exit_action a);
-void invoke_exit_action(unsigned int code);
+[[noreturn]] void invoke_exit_action(unsigned int code);
 
 #include "util/error_codes.h"
 #include "util/warning.h"
@@ -92,16 +92,10 @@ bool is_debug_enabled(const char * tag);
         INVOKE_DEBUGGER();                                                       \
     })
 
-#ifdef __clang__
-#define __compiler_unreachable __builtin_unreachable()
-#else
-#define __compiler_unreachable
-#endif
-
 #ifdef Z3DEBUG
-# define UNREACHABLE() DEBUG_CODE(notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); INVOKE_DEBUGGER(); __compiler_unreachable;)
+# define UNREACHABLE() DEBUG_CODE(notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); INVOKE_DEBUGGER();)
 #else
-# define UNREACHABLE() { notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); invoke_exit_action(ERR_UNREACHABLE); }; __compiler_unreachable
+# define UNREACHABLE() { notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); invoke_exit_action(ERR_UNREACHABLE); } ((void) 0)
 #endif
 
 #ifdef Z3DEBUG