[DependencyScan][Caching] Do not mix CAS instance
Make sure the CAS instances are not mixed during dependency scanning,
especially when used from libSwiftScan C APIs.
For clang scanning service and file system, it should always be created
from the global CAS instance.
For CAS instance inside the swift instance when performing dependency
scanning, it reuse the CAS instance from global as well if that is
already initialized.
rdar://173703843
