Suppose module 'Foo' exists in the search paths and specifies user module version '1.0'.
If the first encountered 'canImport' query is unversioned:
...
Followed by a versioned one:
...
The success of the first check will record an unversioned successful canImport, which will cause the second check to evaluate to 'true', which is incorrect.
This change causes even unversioned 'canImport' checks to track and record the discovered user module version.
Specifically, when the scanner found a candidate which does not carry a user-specified version, it will pass '-module-can-import Foo' to compilation. During compilation, if the check is versioned but the candidate is unversioned, evaluate the check to 'true' to restore the behavior we had with implicitly-built modules.
Resolves rdar://148134993
Teach dependency scanner to report all the module canImport check result
to swift-frontend, so swift-frontend doesn't need to parse swiftmodule
or parse TBD file to determine the versions. This ensures dependency
scanner and swift-frontend will have the same resolution for all
canImport checks.
This also fixes two related issues:
* Previously, in order to get consistant results between scanner and
frontend, scanner will request building the module in canImport check
even it is not imported later. This slightly alters the definition of
the canImport to only succeed when the module can be found AND be
built. This also can affect the auto-link in such cases.
* For caching build, the location of the clang module is abstracted away
so swift-frontend cannot locate the TBD file to resolve
underlyingVersion.
rdar://128067152
