Revert "[SILGen] Fix the type of closure thunks that are passed const referen…"

This reverts commit e074426058.

include/swift/SIL/SILDeclRef.h

@@ -32,10 +32,6 @@ namespace llvm {
   class raw_ostream;
 }
 
-namespace clang {
-class Type;
-}
-
 namespace swift {
   enum class EffectsKind : uint8_t;
   class AbstractFunctionDecl;
@@ -208,9 +204,6 @@ struct SILDeclRef {
                const GenericSignatureImpl *, CustomAttr *>
       pointer;
 
-  // Type of closure thunk.
-  const clang::Type *thunkType = nullptr;
-
   /// Returns the type of AST node location being stored by the SILDeclRef.
   LocKind getLocKind() const {
     if (loc.is<ValueDecl *>())
@@ -264,10 +257,11 @@ struct SILDeclRef {
   ///   for the containing ClassDecl.
   /// - If 'loc' is a global VarDecl, this returns its GlobalAccessor
   ///   SILDeclRef.
-  explicit SILDeclRef(Loc loc, bool isForeign = false,
-                      bool isDistributed = false,
-                      bool isDistributedLocal = false,
-                      const clang::Type *thunkType = nullptr);
+  explicit SILDeclRef(
+      Loc loc,
+      bool isForeign = false,
+      bool isDistributed = false,
+      bool isDistributedLocal = false);
 
   /// See above put produces a prespecialization according to the signature.
   explicit SILDeclRef(Loc loc, GenericSignature prespecializationSig);