[clang] [llvm] [NFC][win] Use an enum for the cfguard module flag (PR #176461)

[Aiden Grossman via cfe-commits]

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@@ -1123,10 +1123,14 @@ void CodeGenModule::Release() {
   }
   if (CodeGenOpts.ControlFlowGuard) {
     // Function ID tables and checks for Control Flow Guard (cfguard=2).
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boomanaiden154 wrote:

`cfguard=` here could probably be updated with the name of the enum?

https://github.com/llvm/llvm-project/pull/176461