Update 3C documentation

[dtarditi]

• The 3C documentation at https://github.com/microsoft/checkedc-clang/blob/master/clang/docs/checkedc/3C/README.md is out-of-date. 3C is now integrated into the checkedc-clang repo, so there is no repo to choose from. It will also be built as part of building checkedc-clang.
• The build directions should be updated too: https://github.com/microsoft/checkedc-clang/blob/master/clang/docs/checkedc/3C/INSTALL.md.
• 3C should be mentioned on the main Checked C wiki at https://github.com/checkedc/checkedc/wiki.