meshletcodec: Fix clang-cl build warning

clang-cl pretends that it's MSVC by defining _MSC_VER but does not
support msvc::flatten, resulting in a build warning and in a potential
codegen regression. It does support __attribute__((flatten)) so simply
make the MSVC detection more precise here.

Author: zeux

src/meshletcodec.cpp

@@ -30,7 +30,7 @@
 #define SIMD_NEON
 #endif
 
-#if defined(_MSC_VER) && _MSC_VER > 1930
+#if defined(_MSC_VER) && !defined(__clang__) && _MSC_VER > 1930
 #define SIMD_FLATTEN [[msvc::flatten]]
 #elif defined(__GNUC__) || defined(__clang__)
 #define SIMD_FLATTEN __attribute__((flatten))