LTC Out-of-tree JIT linkage failure

[henrytwo]

Currently LTC fails to build using the OOT CI config due to this linkage error: https://github.com/llvm/torch-mlir/runs/7549301528?check_suite_focus=true

Currently we work around this by setting TORCH_MLIR_ENABLE_LTC=OFF in cmake, which excludes LTC from compilation and generates a "fake" library in its place.

Additionally, the library type for TorchMLIRJITIRImporter is set to MODULE instead of SHARED. It looks like changing the module type to SHARED is the trigger for the aforementioned CI problems. However, we need this library to be SHARED in order to link with LTC.