[Driver] Fix "unannotated fall-through between switch labels". NFC

Just a simple fix of Werror problem after r367165.

llvm-svn: 367177

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3567,6 +3567,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       case llvm::Triple::thumb:
       case llvm::Triple::thumbeb:
         Args.ClaimAllArgs(options::OPT_mimplicit_it_EQ);
+        break;
       default:
         break;
       }