Fix build failure in DXILResource.h on newer versions of MSVC (#139309)

The change #137258 introduced a build break on newer versions of MSVC:

```
llvm\include\llvm\Analysis\DXILResource.h(674) : warning C4715: 'llvm::DXILResourceBindingInfo::getBindingSpaces': not all control paths return a value
```

Fix is to add a `default` case that will ICE.

Author: dpaoliello

llvm/include/llvm/Analysis/DXILResource.h

@@ -691,6 +691,8 @@ class DXILResourceBindingInfo {
     case dxil::ResourceClass::Sampler:
       return SamplerSpaces;
     }
+
+    llvm_unreachable("Invalid resource class");
   }
 
   friend class DXILResourceBindingAnalysis;