Fix canImport submodule checking in loaders not supporting submodules

[ApolloZhu]

Changes all module loaders that cannot load submodules to return false when checking if it canImport a submodule. CompilerInstance::setUpModuleLoaders in Frontend.cpp mentions that there are 6 types of loaders, and they are all fixed with changes in this PR (or don't need to be fixed):

• SourceLoader (already working, but changed to use hasSubmodule() for consistency)
• MemoryBufferSerializedModuleLoader
• ExplicitSwiftModuleLoader
• ModuleInterfaceLoader
• ImplicitSerializedModuleLoader (handled in SerializedModuleLoaderBase)
• ClangImporter (already working, no change)

Why?

As implemented in #34094, when checking canImport(CoreImage.Anything), because CoreImage is a serialized framework, SerializedModuleLoader checked if it can import CoreImage (ignoring submodule path) and incorrectly return true. I discovered this issue after trying out the latest toolchain, thus this fix + a test involving a submodule in a mixed framework (where multiple module loaders can import the top module).

I ran the new test using the unfixed implementation, and indeed it doesn't pass:

$ swift --version
Apple Swift version 5.6-dev (LLVM 031406b6cea8006, Swift 9f7d3fccad66d76)
Target: x86_64-apple-macosx12.0
$ swift-frontend -typecheck -verify -disable-objc-attr-requires-foundation-module can_import_submodule_in_framework.swift -F .
can_import_submodule_in_framework.swift:23:8: error: expected error not produced
    // expected-error@-1 {{cannot find 'fromSubmodule' in scope}}
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
zsh: exit 1     swift-frontend -typecheck -verify  can_import_submodule_in_framework.swift -F

[WowbaggersLiquidLunch]

@swift-ci please smoke test

[ApolloZhu]

@CodaFi would you mind taking a look at this when you have time?

[CodaFi]

@swift-ci please smoke test

[CodaFi]

@swift-ci test

[ApolloZhu]

Seems that the tests have passed and this is still mergeable. Let me know if there are additional changes needed.

[ApolloZhu]

@CodaFi do you think it's possible for us to merge this before the Swift 5.7 cut-off date (April 4th), so we can ship a less buggy version in that release?

[ApolloZhu]

Rebased on top of main (old history in https://github.com/ApolloZhu/swift/tree/fix-canImport-submodule-checking-backup), tested locally and everything still works, hopefully now Windows platform CI check won't fail

[xwu]

@swift-ci test

[ApolloZhu]

CI status update: all existing checks have passed

[xwu]

cc @beccadax as the last person to work on some of these files, in case there's other, more appropriate reviewers.

[ApolloZhu]

Created #42179 for potential inclusion in the Swift 5.7 release.

[DougGregor]

@swift-ci please smoke test and merge

[DougGregor]

The change looks good to me. Kicking off another CI round to merge it in, thank you!