[clang][deps] Make sure ScanInstance outlives collector

The `ScanInstance` is a local variable in `DependencyScanningAction::runInvocation()` that is referenced by `ModuleDepCollector`. Since D132405, `ModuleDepCollector` can escape the function and can outlive its `ScanInstance`. This patch fixes that.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D133988