Also fix the old definition by verifying that the condition is contextually convertible to bool.
The new C++20 definition reduces codegen bloat and guarantees fast performance while still retaining the expression (so that unused-variable warnings don't trigger).
As an example where this makes a difference, compare the following snippet under `-DABSL_INTERNAL_CPLUSPLUS_LANG=202002`: https://godbolt.org/z/hjf59n84v
```
#include <stdlib.h>
template<class T> struct S { S() { abort(); } static S const s; };
template<class T> S<T> const S<T>::s = {};
#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
#define ABSL_ASSERT(expr) (decltype((expr) ? void() : void())())
#else
#define ABSL_ASSERT(expr) (false ? ((expr) ? void() : void()) : void())
#endif
void foo() { ABSL_ASSERT(((void)&S<int>::s, true)); }
```
We see that, in unoptimized builds, code is still generated with the old definition of `ABSL_ASSERT`. Moreover, even under optimizations (with `-O2`), the call to abort() still lingers with the old definition of `ABSL_ASSERT`.
Therefore the extra generated code can affect both compile- and run-time performance.
PiperOrigin-RevId: 681563573
Change-Id: I7fbfadcc7fd198e8e1daf14615c33687f6b23af7
