[clang] [clang] Add clang::preferred_type attribute for bitfields (PR #69104)

[Aaron Ballman via cfe-commits]

================
@@ -7219,6 +7219,31 @@ its underlying representation to be a WebAssembly ``funcref``.
   }];
 }
 
+def PreferredTypeDocumentation : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+This attribute allows adjusting the type of a bit-field in debug information.
+This can be helpful when a bit-field is intended to store an enumeration value,
+but has to be specified as having the enumeration's underlying type in order to
+facilitate compiler optimizations or bit-field packing behavior. Normally, the
+underlying type is what is emitted in debug information, which can make it hard
+for debuggers to know to map a bit-field's value back to a particular enumeration.
+
+.. code-block:: c++
+
+    enum Colors { Red, Green, Blue };
+
+    struct S {
+      [[clang::preferred_type(Colors)]] unsigned ColorVal : 2;
+      [[clang::preferred_type(bool)]] unsigned UseAlternateColorSpace : 1;
+    } s = { Green, false };
+
+Without the attribute, a debugger is likely to display the value `1` for `ColorVal`
+and `0` for `UseAlternateColorSpace`. With the attribute, the debugger may now
+display `Green` and `false` instead.
----------------
AaronBallman wrote:

```suggestion
Without the attribute, a debugger is likely to display the value ``1`` for ``ColorVal``
and ``0`` for ``UseAlternateColorSpace``. With the attribute, the debugger may now
display ``Green`` and ``false`` instead.
```

https://github.com/llvm/llvm-project/pull/69104