[llvm] [Offload] Add `_LAST` variant for generated enumerations (PR #147314)

[Ross Brunton via llvm-commits]

================
@@ -151,6 +151,12 @@ static void ProcessEnum(const EnumRec &Enum, raw_ostream &OS) {
                 Enum.getEnumValNamePrefix());
 
   OS << formatv("} {0};\n", Enum.getName());
+
+  // Add field for the number of variants
+  OS << formatv(TAB_1
----------------
RossBrunton wrote:

Fair, it now looks like this:

```c
typedef enum ol_alloc_type_t {
  /// Host allocation
  OL_ALLOC_TYPE_HOST = 0,
  /// Device allocation
  OL_ALLOC_TYPE_DEVICE = 1,
  /// Managed allocation
  OL_ALLOC_TYPE_MANAGED = 2,
  /// @cond
  OL_ALLOC_TYPE_LAST = 3,
  OL_ALLOC_TYPE_FORCE_UINT32 = 0x7fffffff
  /// @endcond

} ol_alloc_type_t;
```

https://github.com/llvm/llvm-project/pull/147314