Fix Wimplicit-fallthrough warning introduced in rL357466. NFCI.

llvm-svn: 357467

clang/lib/Basic/Targets/PPC.h

@@ -188,6 +188,7 @@ public:
       // Don't use floating point registers on soft float ABI.
       if (FloatABI == SoftFloat)
         return false;
+      LLVM_FALLTHROUGH;
     case 'b': // Base register
       Info.setAllowsRegister();
       break;