[openmp][runtime] Silence warnings

This fixes several of those when building with MSVC on Windows:
```
[3625/7617] Building CXX object
projects\openmp\runtime\src\CMakeFiles\omp.dir\kmp_affinity.cpp.obj
C:\src\git\llvm-project\openmp\runtime\src\kmp_affinity.cpp(2637):
warning C4062: enumerator 'KMP_HW_UNKNOWN' in switch of enum 'kmp_hw_t'
is not handled
C:\src\git\llvm-project\openmp\runtime\src\kmp.h(628): note: see
declaration of 'kmp_hw_t'
```

Author: aganea

openmp/runtime/src/kmp_affinity.cpp

@@ -2634,8 +2634,9 @@ static int __kmp_topology_type_2_intel_type(kmp_hw_t type) {
     return INTEL_LEVEL_TYPE_MODULE;
   case KMP_HW_DIE:
     return INTEL_LEVEL_TYPE_DIE;
+  default:
+    return INTEL_LEVEL_TYPE_INVALID;
   }
-  return INTEL_LEVEL_TYPE_INVALID;
 }
 
 struct cpuid_level_info_t {