[clang] [Clang] Implement the core language parts of P2786 - Trivial relocation (PR #127636)

[via cfe-commits]

================
@@ -1875,6 +1875,54 @@ static ExprResult BuiltinIsWithinLifetime(Sema &S, CallExpr *TheCall) {
         << 0;
     return ExprError();
   }
+  return TheCall;
+}
+
+static ExprResult BuiltinTriviallyRelocate(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCount(TheCall, 3))
+    return ExprError();
+
+  QualType Dest = TheCall->getArg(0)->getType();
+  if (!Dest->isPointerType() || Dest.getCVRQualifiers() != 0) {
+    S.Diag(TheCall->getArg(0)->getExprLoc(),
+           diag::err_builtin_trivially_relocate_invalid_arg_type)
+        << /*a pointer*/ 0;
+    return ExprError();
+  }
+
+  QualType T = Dest->getPointeeType();
+  if (S.RequireCompleteType(TheCall->getBeginLoc(), T,
+                            diag::err_incomplete_type))
+    return ExprError();
+
+  if (T.isConstQualified() ||
+      !T.isCppTriviallyRelocatableType(S.getASTContext())) {
+    S.Diag(TheCall->getArg(0)->getExprLoc(),
+           diag::err_builtin_trivially_relocate_invalid_arg_type)
+        << (T.isConstQualified() ? /*non-const*/ 1 : /*relocatable*/ 2);
+    return ExprError();
+  }
+
+  TheCall->setType(Dest);
+
+  QualType Src = TheCall->getArg(1)->getType();
+  if (Src.getCanonicalType() != Dest.getCanonicalType()) {
+    S.Diag(TheCall->getArg(0)->getExprLoc(),
----------------
Sirraide wrote:

```suggestion
    S.Diag(TheCall->getArg(1)->getExprLoc(),
```
Do we maybe want to point to the location of the `src` argument instead of the `dest` argument here?

https://github.com/llvm/llvm-project/pull/127636