[NFC][CodeGen] Change CodeGenOpt::Level/CodeGenFileType into enum classes (#66295)

This will make it easy for callers to see issues with and fix up calls
to createTargetMachine after a future change to the params of
TargetMachine.

This matches other nearby enums.

For downstream users, this should be a fairly straightforward
replacement,
e.g. s/CodeGenOpt::Aggressive/CodeGenOptLevel::Aggressive
or s/CGFT_/CodeGenFileType::
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
index 55a63ec..ec23445b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
@@ -295,10 +295,10 @@
   LLVMContext *Context = nullptr;
 
   SelectionDAGBuilder(SelectionDAG &dag, FunctionLoweringInfo &funcinfo,
-                      SwiftErrorValueTracking &swifterror, CodeGenOpt::Level ol)
+                      SwiftErrorValueTracking &swifterror, CodeGenOptLevel ol)
       : SDNodeOrder(LowestSDNodeOrder), TM(dag.getTarget()), DAG(dag),
-        SL(std::make_unique<SDAGSwitchLowering>(this, funcinfo)), FuncInfo(funcinfo),
-        SwiftError(swifterror) {}
+        SL(std::make_unique<SDAGSwitchLowering>(this, funcinfo)),
+        FuncInfo(funcinfo), SwiftError(swifterror) {}
 
   void init(GCFunctionInfo *gfi, AAResults *AA, AssumptionCache *AC,
             const TargetLibraryInfo *li);