[clang] [clang][NFC] Refactor enums that hold size of `Type` and `DeclContext` bit-fields (PR #70296)

[Aaron Ballman via cfe-commits]

================
@@ -1663,11 +1663,12 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     /// Actually an ArrayType::ArraySizeModifier.
     unsigned SizeModifier : 3;
   };
+  enum { NumArrayTypeBits = NumTypeBits + 3 + 3 };
----------------
AaronBallman wrote:

```suggestion
  enum { NumArrayTypeBits = NumTypeBits + 6 };
```

https://github.com/llvm/llvm-project/pull/70296