[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

utils/gyb_sourcekit_support/UIDs.py

@@ -179,6 +179,8 @@ UID_KEYS = [
     KEY('OptimizeForIDE', 'key.optimize_for_ide'),
     KEY('RequiredBystanders', 'key.required_bystanders'),
     KEY('ReusingASTContext', 'key.reusingastcontext'),
+    KEY('CompletionCheckDependencyInterval',
+        'key.completion_check_dependency_interval'),
 ]