[mlir] Add initial graphdef->mlir generation

This CL adds:
  * One graphdef extracted from the TF test suite. More will come.
  * Scaffolding for the "graphdef2mlir" tool.
  * Importing of simple graphs. Type inference is not yet working, and attributes do not work either.
  * A fix for CFGFunction::~CFGFunction to not die if the function was destroyed without a terminator (for example if we exit early due to an error).

PiperOrigin-RevId: 206965992
1 file changed
tree: b823c527ca889046eccc7adccef06fe94c2cf2c9
  1. include/
  2. lib/
  3. test/
  4. tools/
  5. utils/
  6. .clang-format
  7. LICENSE.TXT