Bug#29125 Windows Server X64: so many compiler warnings

- Remove bothersome warning messages.  This change focuses on the warnings 
that are covered by the ignore file: support-files/compiler_warnings.supp.
- Strings are guaranteed to be max uint in length

sql/sql_string.cc

@@ -458,7 +458,7 @@ bool String::append(const char *s,uint32 arg_length)
 
 bool String::append(const char *s)
 {
-  return append(s, strlen(s));
+  return append(s, (uint) strlen(s));
 }
 
 
@@ -1051,7 +1051,7 @@ outp:
       }
     }
     *from_end_pos= from;
-    res= to - to_start;
+    res= (uint) (to - to_start);
   }
   return (uint32) res;
 }