fix: UB casts to unspecified width enums in proto

Found by edef's fuzzing. We::jade don't think this is security-relevant
or likely to be mistreated by a compiler, but UB is bad.

https://eel.is/c++draft/expr.static.cast#8

> If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]), and otherwise, the behavior is undefined

Notably, the range is defined as the smallest bitfield type that could hold all the values, not any real type which exists: https://eel.is/c++draft/dcl.enum#8.

This is basically a footgun, and I'm writing a clang-tidy check to forbid casting to such types. But first I needed to write clang-tidy testing infrastructure: https://gerrit.lix.systems/c/lix/+/5493.

Change-Id: Ieaaa0fe2a92fd9f24f60761e312741186a6a6964

lix/libstore/build-result.hh

@@ -21,7 +21,7 @@ struct BuildResult
      * Therefore, don't remove status codes, and only add new status
      * codes at the end of the list.
      */
-    enum Status {
+    enum Status : uint8_t {
         Built = 0,
         Substituted,
         AlreadyValid,

lix/libstore/gc-store.hh

@@ -39,13 +39,13 @@ struct GCOptions
      *    Any that could not be deleted are returned via the
      *    `kept` field of GCResults.
      */
-    typedef enum {
+    enum GCAction : uint8_t {
         gcReturnLive,
         gcReturnDead,
         gcDeleteDead,
         gcDeleteSpecific,
         gcTryDeleteSpecific,
-    } GCAction;
+    };
 
     GCAction action{gcDeleteDead};
 

lix/libutil/error.hh

@@ -35,8 +35,7 @@
 
 namespace nix {
 
-
-typedef enum {
+enum Verbosity : uint8_t {
     lvlError = 0,
     lvlWarn,
     lvlNotice,
@@ -45,7 +44,7 @@ typedef enum {
     lvlChatty,
     lvlDebug,
     lvlVomit
-} Verbosity;
+};
 
 template<>
 struct json::is_integral_enum<Verbosity> : std::true_type {};