Enable LGTM for code analysis (Semmle) (and Facebook Infer)

Using these tools will help to eliminate obvious errors. For zstd it is important from the security point of view, and C/C++ are supported. See:
- https://lgtm.com/
- https://github.com/marketplace/lgtm

Another tool (from your company, haha) with similar capabilities is [Infer](https://fbinfer.com/), it works with both C and C++, helping to find some bugs that are not overlapping with Coverity or LGTM (Semmle) due to the theory used. It's [open source at GitHub](https://github.com/facebook/infer) too, but please be sure to use the version from master - there are some important bugs for C/C++ code were fixed recently and din't made into the release yet. There are [docker images](https://github.com/facebook/infer/tree/master/docker) as well, to simplify the installation.