This makes it easy to wire up build systems like Bazel that need to know
the actual include paths used. It also gives us a convenient place to
export any other information that build systems or integrations need,
and to get debugging info from users.
Most of the complexity is computing the Clang header search paths, but
I couldn't see a direct way to get closer to the source-of-truth than
this, and it doesn't seem _too_ unreasonable.
Depends on #6636 - start review at commit
[643fdab1](6637/commits/643fdab1)
Chandler Carruth