[ExplicitModule] Fix canImport lookup for swift explicit module build

[cachemeifyoucan]

Previously, canImport lookup is not completely working with explicit module due to two issues:

• For clang modules, canImport check still do a full modulemap lookup which is repeated work from scanner. For caching builds, this lookup cannot be performed because all modulemap and search path are dropped after scanning.
• For swift module, if the canImport module was never actually imported later, this canImport check will fail during the actual compilation, causing different dependencies in the actual compilation.

To fix the problem, first unified the lookup method for clang and swift module, which will only lookup the module dependencies reported by scanner to determine if canImport succeed or not. Secondly, add all the successful canImport check modules into the dependency of the current module so this information can be used during actual compilation.

Note the behavior change here is that if a module is only checked in canImport but never imported still needs to be built. Comparing to implicit module build, this can bring in additional clang modules if they are only check inside canImport but should not increase work for swift modules (where binary module needs to be on disk anyway) or the most common usecase for canImport which is to check the same module before importing.

rdar://121082031

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

@swift-ci please smoke test linux platform

[cachemeifyoucan]

Note: this is not enough because you can write a much more complicated if clause for canImport. The only way that doesn't duplicate the work is to ask parser to remember all the checks. Updating the patch.

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

Note: this is not enough because you can write a much more complicated if clause for canImport. The only way that doesn't duplicate the work is to ask parser to remember all the checks. Updating the patch.

Fixed now and shouldn't not add any scanning overhead.

[artemcm]

This looks good. Thanks for the fix!

[benlangmuir]

Nit: I think this could be rewritten as a template file using split-file and then pipe it through sed to inject the strings you need. But I don't want to block the change on this.

[cachemeifyoucan]

I think it might be useful to have a python script to generate this file if this keeps happening. sed doesn't work well here either for how many places need to be replaced.