Add script to convert from TFLite to C++ using EmitC.

[jacob-hegna]

To get from TF saved model to TFLite, see:
llvm/lib/Analysis/models/saved-model-to-tflite.py.

Example usage:

python compiler_opt/tools/tflite_to_cpp.py \
  --name="InlineOzTestModel" \
  --emitc_runtime_path=/tmp/mlir-emitc/reference-implementation/include/emitc
  --iree_import_tflite_path=/tmp/iree-import-tflite \
  --input=/tmp/model.tflite \
  --emitc_opt_path=/tmp/emitc-opt \
  --mlir_translate_path=/tmp/mlir-translate \
  --output_dir=/tmp/ \
  --base_class=::llvm::MLInlineOzEmitCModel \
  --additional_headers="llvm/Analysis/MLInlineEmitCModel.h"

[jacob-hegna]

will upload tests early next week. uploaded the patch without them initially so that reviewer(s) of reviews.llvm.org/D146483 can reference the code that produces the EmitC models.

[mtrofin]

Why can't this go in llvm/llvm-project/llvm/lib/Analysis/models?