Fixed all clang -Wsigned-enum-bitfield warnings

Made enums involved in bitfields unsigned.

Due to a bug, gcc < 9.3 warns about bitfields not being big enough to hold the enum, even though they are. So keep the plain enum for old gcc.

include/fmt/core.h

@@ -2029,14 +2029,20 @@ template <typename Context> class basic_format_args {
 // between clang and gcc on ARM (#1919).
 using format_args = basic_format_args<format_context>;
 
-// We cannot use enum classes as bit fields because of a gcc bug
+// We cannot use enum classes as bit fields because of a gcc bug before 9.3
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61414.
+#if FMT_GCC_VERSION && FMT_GCC_VERSION < 903
+# define FMT_ENUM_UNDERLYING_TYPE(type)
+#else
+# define FMT_ENUM_UNDERLYING_TYPE(type) : type
+#endif
+
 namespace align {
-enum type { none, left, right, center, numeric };
+enum type FMT_ENUM_UNDERLYING_TYPE(unsigned char) { none, left, right, center, numeric };
 }
 using align_t = align::type;
 namespace sign {
-enum type { none, minus, plus, space };
+enum type FMT_ENUM_UNDERLYING_TYPE(unsigned char) { none, minus, plus, space };
 }
 using sign_t = sign::type;