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.```

1 files changed, 15 insertions, 2 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 500d43a..c09afef9 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -5975,9 +5975,22 @@ static bool CheckConstexprFunction(EvalInfo &Info, SourceLocation CallLoc,
                                         Definition->hasAttr<MSConstexprAttr>())))
     return true;
 
-  if (Info.getLangOpts().CPlusPlus11) {
-    const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  // Special note for the assert() macro, as the normal error message falsely
+  // implies we cannot use an assertion during constant evaluation.
+  if (CallLoc.isMacroID() && DiagDecl->getIdentifier()) {
+    // FIXME: Instead of checking for an implementation-defined function,
+    // check and evaluate the assert() macro.
+    StringRef Name = DiagDecl->getName();
+    bool AssertFailed =
+        Name == "__assert_rtn" || Name == "__assert_fail" || Name == "_wassert";
+    if (AssertFailed) {
+      Info.FFDiag(CallLoc, diag::note_constexpr_assert_failed);
+      return false;
+    }
+  }
 
+  if (Info.getLangOpts().CPlusPlus11) {
     // If this function is not constexpr because it is an inherited
     // non-constexpr constructor, diagnose that directly.
     auto *CD = dyn_cast<CXXConstructorDecl>(DiagDecl);