[clang] [llvm] [DebugInfo] Use DW_op_bit_piece for structured bindings of bitfields (PR #85665)

[Adrian Prantl via cfe-commits]

================
@@ -8,8 +8,8 @@ struct S0 {
 // CHECK-LABEL: define dso_local void @_Z3fS0v
 // CHECK:                        alloca %struct.S0, align 4
 // CHECK-NEXT:    [[TMP0:%.*]] = alloca %struct.S0, align 4
-// CHECK:         call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_A:![0-9]+]], metadata !DIExpression())
-// CHECK-NEXT:    call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_B:![0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, 2))
+// CHECK:         call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_A:![0-9]+]], metadata !DIExpression(DW_OP_bit_piece, 16, 0))
----------------
adrian-prantl wrote:

```
  %0 = alloca %struct.anon, align 4
  store i32 0, ptr %retval, align 4
  tail call void @llvm.dbg.declare(metadata ptr %0, metadata !23, metadata !DIExpression()), !dbg !25
  tail call void @llvm.dbg.declare(metadata ptr %0, metadata !26, metadata !DIExpression(DW_OP_plus_uconst, 1)), !dbg !27

...
!23 = !DILocalVariable(name: "a", scope: !18, file: !5, line: 2, type: !24)
!26 = !DILocalVariable(name: "b", scope: !18, file: !5, line: 2, type: !24)
```

https://github.com/llvm/llvm-project/pull/85665