Move FMT_CLANG_VERSION definition to core.h (#1568)

Previously format.h defined FMT_CLANG_VERSION after including core.h, however core.h tests FMT_CLANG_VERSION when it defines FMT_API.

include/fmt/core.h

@@ -36,6 +36,12 @@
 #  define FMT_HAS_CPP_ATTRIBUTE(x) 0
 #endif
 
+#ifdef __clang__
+#  define FMT_CLANG_VERSION (__clang_major__ * 100 + __clang_minor__)
+#else
+#  define FMT_CLANG_VERSION 0
+#endif
+
 #if defined(__GNUC__) && !defined(__clang__)
 #  define FMT_GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
 #else

include/fmt/format.h

@@ -43,12 +43,6 @@
 
 #include "core.h"
 
-#ifdef __clang__
-#  define FMT_CLANG_VERSION (__clang_major__ * 100 + __clang_minor__)
-#else
-#  define FMT_CLANG_VERSION 0
-#endif
-
 #ifdef __INTEL_COMPILER
 #  define FMT_ICC_VERSION __INTEL_COMPILER
 #elif defined(__ICL)