Fixed macro redefinition warnings when compiling with clang-cl.

Both clang-cl and Clang/C2 #define _MSC_VER but also have support for __builtin_clz and __builtin_clzll, leading to duplicate macro definition warnings. This change suppresses emulation of clz using _BitScanReverse if the __clang__ macro is defined.

format.h

@@ -75,7 +75,7 @@ typedef __int64          intmax_t;
 # define FMT_API
 #endif
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 # include <intrin.h>  // _BitScanReverse, _BitScanReverse64
 
 namespace fmt {