Add a page to track C defect report status

We currently have a page for tracking defects against the C++ standard,
but we don't have the same information for the C standard. This starts
us down the path of being able to track that in a way our users can see.

There are *a lot* of entries marked as "Unknown". As we validate
Clang's behavior for a given DR by adding a test case for it, we can
slowly begin to improve this page over time.

This page is now linked from the existing C status page, which was
updated slightly as a result of these changes as well.

Note, unlike with the C++ defect report page, this content is not auto-
generated from a source document and is not automatically updated from
test comments. It may be worthwhile to automate the updates based on
our test coverage, but that can happen later.