[gardening][enum class] Replace unsigned char with uint8_t for consistency

Before the changes:

- `git grep -E "enum class .+ : uint8_t \{" | wc -l`: 90
- `git grep -E "enum class .+ : unsigned char \{" | wc -l`: 26

lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp

@@ -191,11 +191,7 @@ static void InsertCFGDiamond(SILValue Cond, SILLocation Loc, SILBuilder &B,
 //===----------------------------------------------------------------------===//
 
 namespace {
-  enum class DIKind : unsigned char {
-    No,
-    Yes,
-    Partial
-  };
+enum class DIKind : uint8_t { No, Yes, Partial };
 } // end anonymous namespace
 
 /// This implements the lattice merge operation for 2 optional DIKinds.