[clang] [CIR] Add support for indirect calls (PR #139748)

[Andy Kaylor via cfe-commits]

================
@@ -1839,16 +1837,24 @@ class CIR_CallOpBase<string mnemonic, list<Trait> extra_traits = []>
   // the upstreaming process moves on. The verifiers is also missing for now,
   // will add in the future.
 
-  dag commonArgs = (ins FlatSymbolRefAttr:$callee,
-                        Variadic<CIR_AnyType>:$args);
+  dag commonArgs = (ins OptionalAttr<FlatSymbolRefAttr>:$callee,
+      Variadic<CIR_AnyType>:$args);
 }
 
 def CallOp : CIR_CallOpBase<"call", [NoRegionArguments]> {
   let summary = "call a function";
   let description = [{
-    The `cir.call` operation represents a direct call to a function that is
-    within the same symbol scope as the call. The callee is encoded as a symbol
-    reference attribute named `callee`.
+    The `cir.call` operation represents a function call. It could represent
+    either a direct call or an indirect call.
+
+    If the operation represents a direct call, the callee should be defined
+    within the same symbol scope as the call. The `callee` attribute contains a
+    symbol reference to the callee function. All operands of this operation are
+    arguments to the callee function.
+
+    If the operation represents an indirect call, the `callee` attribute is
+    empty. The first operand of this operation must be a pointer to the callee
+    function. All the rest operands are arguments to the callee function.
----------------
andykaylor wrote:

```suggestion
    function. The rest operands are arguments to the callee function.
```

https://github.com/llvm/llvm-project/pull/139748