[AST] Fix a -Wimplicit-fallthrough warning in ScanfFormatString.cpp

[riccibruno]

Summary

This is showing up with some bots
(eg http://lab.llvm.org:8011/builders/clang-atom-d525-fedora-rel/builds/19061/steps/build%20stage%201/logs/warnings%20%284%29)

I am putting this for review since I am not familiar with this code and am not
sure whether adding llvm_unreachable is better than adding a default case
covering the invalid cases, as is done a few lines below.

[hans]

Sorry for the late reply; I was on vacation. It seems Erich Keane already fixed this in r350941.

[riccibruno]

Nice, thanks!

lib/AST/ScanfFormatString.cpp

         case LengthModifier::AsWide:
           return ArgType::Invalid();
       }
+      llvm_unreachable("Unexpected length modifier!");
 
     // Unsigned int.
     case ConversionSpecifier::oArg:

         case LengthModifier::AsWide:
           return ArgType::Invalid();
       }
+      llvm_unreachable("Unexpected length modifier!");
 
     // Float.
     case ConversionSpecifier::aArg: