-Wformat-non-iso: warn about positional arguments (pr12017)

This renames the -Wformat-non-standard flag to -Wformat-non-iso,
rewords the current warnings a bit (pointing out that a format string
is not supported by ISO C rather than being "non standard"),
and adds a warning about positional arguments.

llvm-svn: 152403

clang/lib/Analysis/FormatString.cpp

@@ -156,6 +156,9 @@ clang::analyze_format_string::ParseArgPosition(FormatStringHandler &H,
   }
 
   if (Amt.getHowSpecified() == OptionalAmount::Constant && *(I++) == '$') {
+    // Warn that positional arguments are non-standard.
+    H.HandlePosition(Start, I - Start);
+
     // Special case: '%0$', since this is an easy mistake.
     if (Amt.getConstantAmount() == 0) {
       H.HandleZeroPosition(Start, I - Start);