Check if dependencies are modified since the last checking.
Dependencies:
- Other source files in the current module
- Dependent files collected by the dependency tracker
When:
- If the last dependency check was over N (defaults to 5) seconds ago
Invalidate if:
- The dependency file is missing
- The modification time of the dependecy is greater than the last check
- If the modification time is zero, compare the content using the file
system from the previous completion and the current completion
rdar://problem/62336432
