Fix MSVC "not all control paths return a value" warning. NFC.

Author: RKSimon

llvm/lib/Target/AArch64/AArch64SLSHardening.cpp

@@ -131,6 +131,7 @@ class ThunksSet {
     case ThunkKind::ThunkBRAB:
       return BLRABThunks[indexOfXReg(Xm)];
     }
+    llvm_unreachable("Unknown ThunkKindId enum");
   }
 };