[clang] [Mips] Convert -mnan=legacy to nan2008 when architecture support nan2008 (PR #153777)

[Simon Pilgrim via cfe-commits]

================
@@ -620,6 +620,10 @@ def warn_target_unsupported_nan2008 : Warning<
 def warn_target_unsupported_nanlegacy : Warning<
   "ignoring '-mnan=legacy' option because the '%0' architecture does not support it">,
   InGroup<UnsupportedNan>;
+def warn_target_unsupported_convertnanlegacytonan2008 : Warning<
+  "ignoring '-mnan=legacy' option because now we does not support it and set to `-mnan=2008` "
+  "option because the '%0' architecture support it">,
+  InGroup<UnsupportedNan>;
----------------
RKSimon wrote:

Is this any better?
```
"ignoring unsupported '-mnan=legacy' option and instead set to `-mnan=2008` option because the '%0' architecture supports it"
```

https://github.com/llvm/llvm-project/pull/153777