Feature request: Super fast output of all warnings

For [SublimeHaskell](https://github.com/SublimeHaskell/SublimeHaskell) (and potentially all other editors and similar tools) we need to quickly be able to list all warnings of a cabal project.

`cabal build` does not show warnings on already compiled modules, so we currently use

```
cabal build --ghc-options="-fforce-recomp -Wall -fno-code"
```

to quickly get all warnings of the project. This is obviously slow since all code has to be re-compiled (even though code generation can be skipped).

It would be nice if cabal could use its knowledge of which code has no changes to maintain a cache of warnings and error messages that can be queried super quickly.

We could then add an option `cabal build --check` that only typechecks the project and prints out all errors/warnings of unchanged files from the cache, so that a no-op typecheck with warnings is instant and can give feedback very quickly or even as you type.

No files, tests, or entry points are named. Start by tracing how `cabal build` decides whether modules are rebuilt and where warnings and errors are produced; done means a cache-backed check mode can report diagnostics for unchanged modules without recompiling them.

Written by the indexing model from the issue text.