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/mars.h

@@ -514,7 +514,12 @@ void errorSupplemental(Loc loc, const char *format, ...);
 void verror(Loc loc, const char *format, va_list ap, const char *p1 = NULL, const char *p2 = NULL);
 void vwarning(Loc loc, const char *format, va_list);
 void verrorSupplemental(Loc loc, const char *format, va_list);
+
+#if defined(__GNUC__) || defined(__clang__)
+__attribute__((noreturn))
+#endif
 void fatal();
+
 void err_nomem();
 #if IN_LLVM
 void error(const char *format, ...)  IS_PRINTF(1);

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[];