Fix build break in SemaHLSL.cpp on MSVC 2022: warning C4715: 'getResourceClass': not all control paths return a value (#112767)

Moves the existing `llvm_unreachable` statement to the bottom of the
function and changes the case statement to deliberately fall through to
it.

Build break was introduced by #111203

It was not caught by the builders as they use Visual Studio 2019,
whereas this warning only appears in 2022.

---------

Co-authored-by: Matheus Izvekov <mizvekov@gmail.com>

clang/lib/Sema/SemaHLSL.cpp

@@ -102,8 +102,10 @@ static ResourceClass getResourceClass(RegisterType RT) {
     return ResourceClass::Sampler;
   case RegisterType::C:
   case RegisterType::I:
-    llvm_unreachable("unexpected RegisterType value");
+    // Deliberately falling through to the unreachable below.
+    break;
   }
+  llvm_unreachable("unexpected RegisterType value");
 }
 
 DeclBindingInfo *ResourceBindings::addDeclBindingInfo(const VarDecl *VD,