Clean up the code computing the parameters and their types.

This patch harmonizes the steps to build the derived function prototype along
with its parameters. It refactors a lot of tricky code that grew organically
and included many workarounds. This is only a single step in a direction of
merging the common code patterns across visitors.

As a bonus we do not generate broken code when we have derivative parameters
named `_d_x`. That's common in computing hessians.

include/clad/Differentiator/ReverseModeVisitor.h

@@ -682,19 +682,11 @@ namespace clad {
     ///\paramp[in] source An external RMV source
     void AddExternalSource(ExternalRMVSource& source);
 
-    /// Computes and returns the sequence of derived function parameter types.
-    ///
-    /// Information about the original function and the differentiation mode
-    /// are taken from the data member variables.
-    llvm::SmallVector<clang::QualType, 8> ComputeParamTypes(const DiffParams& diffParams);
+    /// Computes and returns the derived function prototype.
+    clang::QualType ComputeDerivativeFunctionType();
 
     /// Builds and returns the sequence of derived function parameters.
-    ///
-    /// Information about the original function, derived function, derived
-    /// function parameter types and the differentiation mode are implicitly
-    /// taken from the data member variables.
-    llvm::SmallVector<clang::ParmVarDecl*, 8>
-    BuildParams(DiffParams& diffParams);
+    void BuildParams(llvm::SmallVectorImpl<clang::ParmVarDecl*>& params);
 
     clang::QualType ComputeAdjointType(clang::QualType T);
     clang::QualType ComputeParamType(clang::QualType T);