[mlir] Fix sporadic build failures due to missing dependency

The build of MLIR occasionally fails (especially on Windows) because there is missing dependency between MLIRLLVMIR and MLIROpenMPOpsIncGen.

1) LLVMDialect.cpp includes LLVMDialect.h
2) LLVMDialect.h includes OpenMPDialect.h
3) OpenMPDialect.h includes OpenMPOpsDialect.h.inc, OpenMPOpsEnums.h.inc and OpenMPOps.h.inc

The OpenMP .inc files are generated by MLIROpenMPOpsIncGen, so MLIRLLVMIR which builds LLVMDialect.cpp should depend on MLIROpenMPOpsIncGen

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D89275