[clang] [CIR] Unblock simple C++ structure support (PR #138368)

[Andy Kaylor via cfe-commits]

================
@@ -365,10 +365,15 @@ mlir::Attribute ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &d) {
   if (!d.hasLocalStorage()) {
     QualType ty = cgm.getASTContext().getBaseElementType(d.getType());
     if (ty->isRecordType())
-      if (d.getInit() && isa<CXXConstructExpr>(d.getInit())) {
-        cgm.errorNYI(d.getInit()->getBeginLoc(),
-                     "tryEmitPrivateForVarInit CXXConstructExpr");
-        return {};
+      if (const CXXConstructExpr *e =
+              dyn_cast_or_null<CXXConstructExpr>(d.getInit())) {
+        const CXXConstructorDecl *cd = e->getConstructor();
----------------
andykaylor wrote:

Classic codegen does assume that `cd` is non-null here. I'm pretty sure that if the `dyn_cast_or_null` gives us a `CXXConstructorExpr` we can count on if having a `CXXConstructorDecl`.

The AST handles any decisions about which constructors are to be called. Currently, the incubator just represents constructors, when needed, as `cir.call` operations. It may be worth doing more than that. In the case of code like this:
```
void f() {
    MyClass obj = func();
}
```
The AST says this:
```
-FunctionDecl <line:12:1, line:14:1> line:12:6 f 'void ()'
  `-CompoundStmt <col:10, line:14:1>
    `-DeclStmt <line:13:5, col:25>
      `-VarDecl <col:5, col:24> col:13 obj 'MyClass' cinit
        `-CallExpr <col:19, col:24> 'MyClass'
          `-ImplicitCastExpr <col:19> 'MyClass (*)()' <FunctionToPointerDecay>
            `-DeclRefExpr <col:19> 'MyClass ()' lvalue Function 0x216ac150 'func' 'MyClass ()'
```
And the incubator generates this CIR:
```
  cir.func @_Z1fv() {
    %0 = cir.alloca !rec_MyClass, !cir.ptr<!rec_MyClass>, ["obj", init] {alignment = 1 : i64}
    %1 = cir.call @_Z4funcv() : () -> !rec_MyClass
    cir.store %1, %0 : !rec_MyClass, !cir.ptr<!rec_MyClass>
    cir.return
  }
```
Is your question whether we should do something here to explicitly show that the copy constructor was elided?

https://github.com/llvm/llvm-project/pull/138368