Bump abc submodule

[povik]

What are the reasons/motivation for this change?

Pulling in berkeley-abc/abc#327; any upstream fixes

[povik]

The CI failure for nix is because abc now wants to pull a googletest dependency from the internet when doing a cmake build but the nix sandbox denies internet access. Cc @widlarizer

[widlarizer]

This isn't a submodule bump, it's switching abc branches away from yosys-experimental. The nix breakage is happening because this PR removes a gtest revert commit that's present on yosys-experimental. I can't track down what this commit even is, since it differs from both possible sources so much:

emil@fridge ~/p/abc (upstream)> git diff --stat 745ea92718c4c861eb08d0120ce1b99805e16839 upstream | tail -n 1
 145 files changed, 814 insertions(+), 9076 deletions(-)
emil@fridge ~/p/abc (upstream)> git diff --stat 745ea92718c4c861eb08d0120ce1b99805e16839 yosys-experimental | tail -n 1
 51 files changed, 348 insertions(+), 3726 deletions(-)

[povik]

This isn't a submodule bump, it's switching abc branches away from yosys-experimental.

This is a bump from 2188bc7 to 745ea92, which is a merge of 2188bc7 and upstream head. I pushed the merge to yosys-experimental for your benefit now.

I can't track down what this commit even is, since it differs from both possible sources so much:

This is what you get in a merge commit...