[mlir] Fix attaching side effects on `FlatSymbolRefAttr`

The names of the generated attribute getters for ops changed some time ago. The method created from the attribute name returns the return type and an additional method of the same name with Attr as suffix is generated which returns the actual attribute as its storage type.

The code generating effects however was using the methods without the Attr suffix, which is a problem in the case of FlatSymbolRefAttr as it has a return type of llvm::StringRef. This would lead to compilation errors as the constructor of SideEffects::EffectInstance expects a SymbolRefAttr in this case.

This patch simply fixes the generated effects code to use the Attr suffixed getter to get the actual storage type of the attribute.

Differential Revision: https://reviews.llvm.org/D117194