[clang] [Clang][ItaniumMangle][NFC] Refactor FunctionTypeDepthState (PR #196240)

[Aaron Ballman via cfe-commits]

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@@ -3784,7 +3776,7 @@ void CXXNameMangler::mangleBareFunctionType(const FunctionProtoType *Proto,
     }
     mangleType(ReturnTy);
 
-    FunctionTypeDepth.leaveResultType();
+    FunctionTypeDepth.leaveFunctionDeclSuffix();
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AaronBallman wrote:

Would it make sense to add an RAII object to do the enter/leave calls? I worry about the possibility of adding a `return` in here someday that breaks this.

https://github.com/llvm/llvm-project/pull/196240