[clang] [TySan] Add initial documentation for Type Sanitizer (PR #123595)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

The language here is a little bit staccato for my taste, but its probably a good enough start.  A few suggestions, else I'm happy whenever @fhahn and the others are.

https://github.com/llvm/llvm-project/pull/123595