Cleanup: Clang warning fixes.

No warnings left on a '-Wall' build except for a single tautological
compare in gen/asm-x86-32.h, which indeed seems like a bug.

dmd2/root/stringtable.h

@@ -33,8 +33,10 @@ private:
     unsigned length;
 
 #ifndef IN_GCC
+#if _MSC_VER
     // Disable warning about nonstandard extension
     #pragma warning (disable : 4200)
+#endif
 #endif
     char lstring[];