[mlir] Fully qualify MemoryEffects

The effect name is used by tablegen when generating the getEffects method of the SideEffectInterfaces. It is currently unqualified even though the class is contained within the mlir namespace, leading to compiler errors when using namespace mlir; isn't used before including the generated cpp file.

This patch fixes that by simply fully qualifying the class name.

Differential Revision: https://reviews.llvm.org/D107171