[flang] Fix standalone builds

In dd875dd8 I added a missing MLIR
dependency in Flang. However, that particular CMake target is not
exported as something available to standalone builds. In this patch is
switch to `MLIRIR` instead, which depends on
`MLIRBuiltinAttributeInterfacesIncGen` - the missing dependency added
previously.

Differential Revision: https://reviews.llvm.org/D120986