[modules] Preprocessing requires compiled header unit modules

[Ben Boeckel <ben.boeckel@kitware.com>]

Hi,

I'm looking to update my patch which implements P1689[1] for GCC. I
originally wrote this back in February 2019 at Kona for a proof of
concept[2], but never really got the time to delve back in until now.
I'd like to get header units implemented prior to submitting it as it
will be an important part in the long run (as I don't think having a
partial implementation in the wild would be beneficial).

Now that modules have been merged, my patch works just fine for named
modules. However, for header unit modules, it runs into a problem that
imported header units are required to be compiled and available in the
mapper while scanning for dependencies.

Example code:

```c++ # use-header.cpp
module;

import "header-unit.hpp";

int main(int argc, char* argv[]) {
    return good;
}
```

```c++ # header-unit.hpp
constexpr int good = 0;
```

When trying to preprocess this:

```console
% g++ -std=c++20 -fmodules-ts -E use-header.cpp -o use-header.cpp.i
In module imported at use-header.cpp:3:1:
./header-unit.hpp: error: failed to read compiled module: No such file or directory
./header-unit.hpp: note: compiled module file is ‘gcm.cache/,/header-unit.hpp.gcm’
./header-unit.hpp: note: imports must be built before being imported
./header-unit.hpp: fatal error: returning to the gate for a mechanical issue
compilation terminated
```

There used to be no need to do this back prior to the modules landing in
`master`, but I can see this being an oversight in the meantime. I am
not familiar enough with the code to know how to untangle its current
state (attempting to bisect the change throws me back in to the depths
of the history without modules so "does it work" is unanswerable there).

I tried this on commits:

    4efde6781bba8d64b9dcff07e7efe71d35aa6f6a c++: Modules Is Landing
    f4ed267fa5b82d6dafbc8afc82baf45bfcae549c master as of the 23rd

Thanks,

--Ben

[1] https://wg21.link/p1689r4
[2] https://github.com/mathstuf/cxx-modules-sandbox/ (see the `docker`
branch for the instructions on putting things together)