Support multi-file compilation, and in particular imports of files from
the prelude, in `carbon language_server`.
In order to properly interface with `CompileDriver`, also switch over to
building a proper VFS from the documents we're given.
Assisted-by: Gemini via Antigravity
Mainly because "sorting_diagnostic_consumer" is legacy, since
`SortingDiagnosticConsumer` became `SortingConsumer`. Also better
reflecting contents of these files.
Where I'm not renaming, I'm less positive about dropping "diagnostics"
from "file_diagnostics" and "null_diagnostics" (which contain both a
consumer and emitter, and "null.h" seems like poor naming), so not doing
that here. Also "diagnostic.h" contains `struct Diagnostic`, so is a
decent fit.
Assisted-by: Google Antigravity with Gemini 3 Flash
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)
Richard Smith
Jon Ross-Perkins
Chandler Carruth