[6.0 🍒][Dependency Scanning] Disable validation of Swift dependency modules' existing pre-built candidate binary module files in the scanner, on a non-caching build.

[artemcm]

• Explanation: [Dependency Scanning] Disable validation of Swift dependency modules' existing pre-built candidate binary module files in the scanner, on a non-caching build. #75556 moved validation of up-to-date binary module dependencies into the dependency scanner, versus being done in interface compilation jobs. This means the scanner now produces "binary module dependency" for discovered textual Swift modules for which an existing up-to-date binary module is found. Unfortunately, this behavior interacts poorly with the incremental build machinery which relied on textual swift dependencies showing up as "textual module dependency" in order to check if they are out-of-date w.r.t. their interface file. Now that such dependencies are only binary modules, the driver's logic conservatively insists on rebuilding their dependents. We need to enhance the driver logic, but in the meantime, simply revert the switch for this logic to disable it for non-caching builds.

• Scope: Compiles using the opt-in Explicit Module Builds feature.

• Risk: Low. This change reverts build behavior for opt-in Explicit Module Builds back to the known build model that was used for a long while prior to [ScanDependency] Move binary module validation into scanner #72291. This prior build model is not unsound, but rather performs validation of up-to-date modules in the interface compilation task, rather than the scanner itself. Paying the price of launching said task which may be a quick no-op.

• Original PR: [Dependency Scanning] Disable validation of Swift dependency modules' existing pre-built candidate binary module files in the scanner, on a non-caching build. #75556

• Testing: Test suite contained existing tests which were modified to exercise both the new default and changed configuration flags for this behavior.

• Issue/Radar: rdar://132340493

• Reviewed By: @cachemeifyoucan

[artemcm]

@swift-ci test

[nkcsgexi]

This PR has fixed a recent incremental build regression. We should take it.