Disable the import of private dependencies when using precise invocat… #9039
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…ions.
ModuleFileSharedCore::getTransitiveLoadingBehavior() has a best-effort mode that is enabled when debugger support is turned on that will try to import implementation-only imports of Swift modules, but won't treat import failures as errors. When explicit modules are on, this has the unwanted side-effect of potentially triggering an implicit Clang module build if one of the internal dependencies of a library was not used to build the target.
I previously fixed this problem by turning off implicit modules while loading the context dependencies. However, we discovered that the transitive loading of private dependencies can also lead to additional Swift modules being pulled in that through their dependencies can lead to dependency cycles that were not a problem at build time.
Therefore, a simpler solution to both problem is turning of private dependency import altogether when using precise compiler invocations.
Note that private dependencies can still sneak into a context via type reconstruction, which can also trigger module imports.
rdar://132360374