Add dot-cfg-changes style view to the llvm-diff tool

[jamieschmeiser]

Summary

Add an option -dot-cfg=<dir> to llvm-diff so that it uses the dot-cfg-changes
graphical view, building a web-site with either the module differences or the
indicated functions.

Note that this diff builds upon https://reviews.llvm.org/D87202

[jamieschmeiser]

Note that the clang-tidy comment about the non-existent header is because this PR builds upon other PRs that have not been incorporated yet, one of which introduces the header file.