[clang] [CIR] Upstream the initial BlockAddressOp implementation (PR #168151)

[Henrich Lauko via cfe-commits]

================
@@ -4800,4 +4800,37 @@ def CIR_AtomicClearOp : CIR_Op<"atomic.clear"> {
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// BlockAddressOp
+//===----------------------------------------------------------------------===//
+
+def CIR_BlockAddressOp : CIR_Op<"blockaddress", [Pure]> {
----------------
xlauko wrote:

```suggestion
def CIR_BlockAddressOp : CIR_Op<"block_address", [Pure]> {
```

https://github.com/llvm/llvm-project/pull/168151