Fix clang-tidy issues in //common. (#3962)

These likely predate the CI integration for `clang-tidy` runs.

Most of these seem good generally, even though I disabled some with
nolint comments. The multilevel pointer one seems almost like a bug in
the check to detect the specific case of `memcpy`, but otherwise seems
like a solid lint.

common/command_line_test.cpp

@@ -26,12 +26,12 @@ constexpr CommandInfo TestCommandInfo = {
     .help_epilogue = "TODO",
 };
 
-enum class TestEnum {
+enum class TestEnum : int8_t {
   Val1,
   Val2,
 };
 
-enum class TestSubcommand {
+enum class TestSubcommand : int8_t {
   Sub1,
   Sub2,
 };