[CIR][CIRGen][Builtin] Support __builtin_return_address (llvm#1046)

test case is from [Traditional Clang CodeGen test
file](https://github.com/llvm/clangir/blob/723e78afb5ae4fbd000269a057410913ade3ef44/clang/test/CodeGen/2004-02-13-BuiltinFrameReturnAddress.c#L5)

Author: ghehg

clang/include/clang/CIR/Dialect/IR/CIROps.td

@@ -4125,6 +4125,37 @@ def MemChrOp : CIR_Op<"libc.memchr"> {
   let hasVerifier = 0;
 }
 
+//===----------------------------------------------------------------------===//
+// ReturnAddrOp
+//===----------------------------------------------------------------------===//
+
+def ReturnAddrOp : CIR_Op<"return_address"> {
+  let arguments = (ins UInt32:$level);
+  let summary = "The return address of the current function, or of one of its callers";
+  let results = (outs Res<VoidPtr, "">:$result);
+
+  let description = [{
+    Represents call to builtin function ` __builtin_return_address` in CIR. 
+    This builtin function returns the return address of the current function, 
+    or of one of its callers. 
+    The `level` argument is number of frames to scan up the call stack.
+    For instance, value of 0 yields the return address of the current function, 
+    value of 1 yields the return address of the caller of the current function, 
+    and so forth. 
+
+    Examples:
+
+    ```mlir
+    %p = return_address(%level) -> !cir.ptr<!void>
+    ```
+  }];
+
+  let assemblyFormat = [{
+    `(` $level `)` attr-dict
+  }];
+  let hasVerifier = 0;
+}
+
 //===----------------------------------------------------------------------===//
 // StdFindOp
 //===----------------------------------------------------------------------===//

clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp

@@ -3403,8 +3403,14 @@ RValue CIRGenFunction::emitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     llvm_unreachable("BI__builtin_wmemcmp NYI");
   case Builtin::BI__builtin_dwarf_cfa:
     llvm_unreachable("BI__builtin_dwarf_cfa NYI");
-  case Builtin::BI__builtin_return_address:
-    llvm_unreachable("BI__builtin_return_address NYI");
+  case Builtin::BI__builtin_return_address: {
+    mlir::Location loc = getLoc(E->getExprLoc());
+    mlir::Attribute levelAttr = ConstantEmitter(*this).emitAbstract(
+        E->getArg(0), E->getArg(0)->getType());
+    int64_t level = mlir::cast<mlir::cir::IntAttr>(levelAttr).getSInt();
+    return RValue::get(builder.create<mlir::cir::ReturnAddrOp>(
+        loc, builder.getUInt32(level, loc)));
+  }
   case Builtin::BI_ReturnAddress:
     llvm_unreachable("BI_ReturnAddress NYI");
   case Builtin::BI__builtin_frame_address: