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
Previously, when evaluating a `#if canImport(Module, _version: 42)` directive the compiler could diagnose and ignore the directive under the following conditions:
- The associated binary module is corrupt/bogus.
- The .tbd for an underlying Clang module is missing a current-version field.
This behavior is surprising when there is a valid `.swiftinterface` available and it only becomes apparent when building against an SDK with an old enough version of the module that the version in the `.swiftinterface` is too low, making this failure easy to miss. Some modules have different versioning systems for their Swift and Clang modules and it can also be intentional for a distributed binary `.swiftmodule` to contain bogus data (to force the compiler to recompile the `.swiftinterface`) so we need to handle both of these cases gracefully and predictably.
Now the compiler will enumerate all module loaders, ask each of them to attempt to parse the module version and then consistently use the parsed version from a single source. The `.swiftinterface` is preferred if present, then the binary module if present, and then finally the `.tbd`. The `.tbd` is still always used exclusively for the `_underlyingVersion` variant of `canImport()`.
Resolves rdar://88723492
The ClangImporter tests for `#if canImport(..., _underlyingVersion: ...)` do not really depend on a Swift overlay being present for Simple.framework, so remove the overlay related `RUN` steps.
