[CIR] Fix access to bitfields inside a union (#154398)

This PR fixes the access to bitfields inside a union.
Previously, we were using a `getMemberOp` to access the field, but
because it is a union, `getMemberOp` would always use index `0`.

For example, given:
```c
typedef union {
  int x;
  int y : 4;
  int z : 8;
} demo;
```
```mlir
!rec_demo = !cir.record<union "demo" {!s32i, !u8i, !u8i}>
```

In the case of:

```c
d.y = 2;
```

It would generate:

```mlir
cir.get_member %0[0] {name = "y"} : !cir.ptr<!rec_demo> -> !cir.ptr<!s32i>
```

with a return type of `!s32i`, when it should be `!u8i`.
the get_member verifier would detect that the return type does not match
the `y` member.
To fix this, we now use `bitcast` to get the start of the union.

Author: Andres-Salamanca

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -369,9 +369,10 @@ Address CIRGenFunction::getAddrOfBitFieldStorage(LValue base,
                                                  unsigned index) {
   mlir::Location loc = getLoc(field->getLocation());
   cir::PointerType fieldPtr = cir::PointerType::get(fieldType);
-  cir::GetMemberOp sea = getBuilder().createGetMember(
-      loc, fieldPtr, base.getPointer(), field->getName(), index);
   auto rec = cast<cir::RecordType>(base.getAddress().getElementType());
+  cir::GetMemberOp sea = getBuilder().createGetMember(
+      loc, fieldPtr, base.getPointer(), field->getName(),
+      rec.isUnion() ? field->getFieldIndex() : index);
   CharUnits offset = CharUnits::fromQuantity(
       rec.getElementOffset(cgm.getDataLayout().layout, index));
   return Address(sea, base.getAlignment().alignmentAtOffset(offset));

clang/test/CIR/CodeGen/bitfield-union.c

@@ -28,3 +28,44 @@ typedef union {
 
 demo d;
 zero_bit z;
+
+void f() {
+    demo d;
+    d.x = 1;
+    d.y = 2;
+    d.z = 0;
+}
+
+// CIR: #bfi_y = #cir.bitfield_info<name = "y", storage_type = !u8i, size = 4, offset = 0, is_signed = true>
+// CIR: #bfi_z = #cir.bitfield_info<name = "z", storage_type = !u8i, size = 8, offset = 0, is_signed = true>
+
+// CIR:   cir.func no_proto dso_local @f
+// CIR:    [[ALLOC:%.*]] = cir.alloca !rec_demo, !cir.ptr<!rec_demo>, ["d"] {alignment = 4 : i64}
+// CIR:    [[ONE:%.*]] = cir.const #cir.int<1> : !s32i
+// CIR:    [[X:%.*]] = cir.get_member [[ALLOC]][0] {name = "x"} : !cir.ptr<!rec_demo> -> !cir.ptr<!s32i>
+// CIR:    cir.store align(4) [[ONE]], [[X]] : !s32i, !cir.ptr<!s32i>
+// CIR:    [[TWO:%.*]] = cir.const #cir.int<2> : !s32i
+// CIR:    [[Y:%.*]] = cir.get_member [[ALLOC]][1] {name = "y"} : !cir.ptr<!rec_demo> -> !cir.ptr<!u8i>
+// CIR:    [[SET:%.*]] = cir.set_bitfield align(4) (#bfi_y, [[Y]] : !cir.ptr<!u8i>, [[TWO]] : !s32i) -> !s32i
+// CIR:    [[ZERO:%.*]] = cir.const #cir.int<0> : !s32i
+// CIR:    [[Z:%.*]] = cir.get_member [[ALLOC]][2] {name = "z"} : !cir.ptr<!rec_demo> -> !cir.ptr<!u8i>
+// CIR:    [[SET2:%.*]] = cir.set_bitfield align(4) (#bfi_z, [[Z]] : !cir.ptr<!u8i>, [[ZERO]] : !s32i) -> !s32i
+// CIR:    cir.return
+
+// LLVM: define dso_local void @f
+// LLVM:   [[ALLOC:%.*]] = alloca %union.demo, i64 1, align 4
+// LLVM:   store i32 1, ptr [[ALLOC]], align 4
+// LLVM:   [[BFLOAD:%.*]] = load i8, ptr [[ALLOC]], align 4
+// LLVM:   [[CLEAR:%.*]] = and i8 [[BFLOAD]], -16
+// LLVM:   [[SET:%.*]] = or i8 [[CLEAR]], 2
+// LLVM:   store i8 [[SET]], ptr [[ALLOC]], align 4
+// LLVM:   store i8 0, ptr [[ALLOC]], align 4
+
+// OGCG: define dso_local void @f
+// OGCG:   [[ALLOC:%.*]] = alloca %union.demo, align 4
+// OGCG:   store i32 1, ptr [[ALLOC]], align 4
+// OGCG:   [[BFLOAD:%.*]] = load i8, ptr [[ALLOC]], align 4
+// OGCG:   [[CLEAR:%.*]] = and i8 [[BFLOAD]], -16
+// OGCG:   [[SET:%.*]] = or i8 [[CLEAR]], 2
+// OGCG:   store i8 [[SET]], ptr [[ALLOC]], align 4
+// OGCG:   store i8 0, ptr [[ALLOC]], align 4