Better diagnostics when assertion fails in `consteval` (#130458)

Take this piece of code:
```cpp
#include <cassert>

consteval int square(int x) {
  int result = x * x;
  assert(result == 42);
  return result;
}

void test() {
  auto val = square(2);
}
```
The assertion will fail, and `clang++` will output
(https://godbolt.org/z/hjz3KbTTv):
```cpp
<source>:10:14: error: call to consteval function 'square' is not a constant expression
   10 |   auto val = square(2);
      |              ^
<source>:5:3: note: non-constexpr function '__assert_fail' cannot be used in a constant expression
    5 |   assert(result == 42);
      |   ^
/usr/include/assert.h:95:9: note: expanded from macro 'assert'
   95 |       : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
      |         ^
<source>:10:14: note: in call to 'square(2)'
   10 |   auto val = square(2);
      |              ^~~~~~~~~
/usr/include/assert.h:69:13: note: declared here
   69 | extern void __assert_fail (const char *__assertion, const char *__file,
      |             ^
1 error generated.
Compiler returned: 1
```
This is confusing because it implies that the issue was using an
assertion in a constant-evaluted context, and not that the assertion
failed (`assert()` is OK in constant evaluation). This PR changes the
error message to:
```cpp
test.cpp:10:14: error: call to consteval function 'square' is not a constant expression
   10 |   auto val = square(2);
      |              ^
test.cpp:5:3: note: assertion failed in consteval context: 'result == 42'
    5 |   assert(result == 42);
      |   ^
/nix/store/lw21wr626v5sdcaxxkv2k4zf1121hfc9-glibc-2.40-36-dev/include/assert.h:102:9: note: expanded from macro 'assert'
  102 |       : __assert_fail (#expr, __ASSERT_FILE, __ASSERT_LINE,             \
      |         ^
test.cpp:10:14: note: in call to 'square(2)'
   10 |   auto val = square(2);
      |              ^~~~~~~~~
1 error generated.```

0 files changed, 0 insertions, 0 deletions