[CodeCompletion] Give up fast-completion if dependent files are modified

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

test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src/Funcs.swift

@@ -0,0 +1,2 @@
+public func swiftFWFunc() -> Int { return 1 }
+