Ignore unrecognized files in CompilerProvider

Summary:
Sometimes the compiler provider is called on files that we don't
compile. Instead of complaining, we just return an empty set of includes
or defines.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8653