[clang] [CIR] Emit classic's attributes for a non-byval indirect argument (PR #222445)

[Adam Smith via cfe-commits]

================
@@ -21,19 +21,20 @@ using namespace mlir::abi;
 
 // This rewrite context supports the Direct (with or without coercion),
 // Extend, Ignore, Indirect-return (sret), Indirect-argument (byval and
-// byref), and Expand (struct flattening) classifications.
+// non-byval), and Expand (struct flattening) classifications.
 //
-// "byref" here is the llvm.byref case of an Indirect argument, not C++
-// pass-by-reference.  A C++ reference parameter is already a pointer by the
-// time this classifier runs, so it classifies Direct.  byref instead means
-// a by-value parameter whose type cannot be copied freely, because it has a
-// non-trivial copy constructor, move constructor, or destructor, so the ABI
-// passes it through a pointer instead of in registers.
+// An Indirect argument is byval or not, following its classification's
----------------
adams381 wrote:

Yes.  Nothing in CIR uses `llvm.byref` after this, so the attribute is free for `IndirectAliased` to mean what it was added to mean.

https://github.com/llvm/llvm-project/pull/222445