util: enforce unreachable()'s argument being a literal string

This prevents the bugs fixed in the previous commits.

Signed-off-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22529>

src/util/macros.h

@@ -122,17 +122,17 @@
 #if defined(HAVE___BUILTIN_UNREACHABLE) || __has_builtin(__builtin_unreachable)
 #define unreachable(str)    \
 do {                        \
-   assert(!str);            \
+   assert(!"" str);         \
    __builtin_unreachable(); \
 } while (0)
 #elif defined (_MSC_VER)
 #define unreachable(str)    \
 do {                        \
-   assert(!str);            \
+   assert(!"" str);         \
    __assume(0);             \
 } while (0)
 #else
-#define unreachable(str) assert(!str)
+#define unreachable(str) assert(!"" str)
 #endif
 
 /**