[clang][CodeGenModule] remove declaration of GetAddrOfConstantString

It looks like the definition was removed in cd21d54 but the
declaration was not.  Surprisingly (to me), that doesn't seem to produce
any kind of diagnostic.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D156182

clang/include/clang/CodeGen/ConstantInitBuilder.h

@@ -41,7 +41,7 @@ class CodeGenModule;
 ///    for (auto &widget : widgets) {
 ///      auto widgetDesc = widgetArray.beginStruct();
 ///      widgetDesc.addInt(CGM.SizeTy, widget.getPower());
-///      widgetDesc.add(CGM.GetAddrOfConstantString(widget.getName()));
+///      widgetDesc.add(CGM.GetAddrOfConstantStringFromLiteral(widget.getName()));
 ///      widgetDesc.add(CGM.GetAddrOfGlobal(widget.getInitializerDecl()));
 ///      widgetDesc.finishAndAddTo(widgetArray);
 ///    }

clang/lib/CodeGen/CodeGenModule.h

@@ -1019,11 +1019,6 @@ class CodeGenModule : public CodeGenTypeCache {
   /// Return a pointer to a constant CFString object for the given string.
   ConstantAddress GetAddrOfConstantCFString(const StringLiteral *Literal);
 
-  /// Return a pointer to a constant NSString object for the given string. Or a
-  /// user defined String object as defined via
-  /// -fconstant-string-class=class_name option.
-  ConstantAddress GetAddrOfConstantString(const StringLiteral *Literal);
-
   /// Return a constant array for the given string.
   llvm::Constant *GetConstantArrayFromStringLiteral(const StringLiteral *E);