[clang] [CIR] Upstream support for references (PR #138001)

[Erich Keane via cfe-commits]

================
@@ -102,3 +102,20 @@ size_type max_size() {
 // CHECK:   %3 = cir.cast(integral, %2 : !s32i), !u64i
 // CHECK:   %4 = cir.const #cir.int<8> : !u64i
 // CHECK:   %5 = cir.binop(div, %3, %4) : !u64i
+
+void ref_arg(int &x) {
+  int y = x;
+  x = 3;
+}
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erichkeane wrote:

Can we also have tests for 'function that returns reference'?
```
ClassTy &foo();
ClassTy &&foo();
```

And perhaps `int &z = x`? 

https://github.com/llvm/llvm-project/pull/138001