[CodeComplete] Add completions for filenames in #include directives.
The dir component ("somedir" in #include <somedir/fo...>) is considered fixed.
We append "foo" to each directory on the include path, and then list its files.
Completions are of the forms:
The filter is set to the filename part ("fo"), so fuzzy matching can be
applied to the filename only.
No fancy scoring/priorities are set, and no information is added to
CodeCompleteResult to make smart scoring possible. Could be in future.
Differential Revision: https://reviews.llvm.org/D52076