Allow additional file suffixes/extensions considered as source in main include grouping

Summary:
By additional regex match, grouping of main include can be enabled in files that are not normally considered as a C/C++ source code.
For example, this might be useful in templated code, where template implementations are being held in *Impl.hpp files.
On the occassion, 'assume-filename' option description was reworded as it was misleading. It has nothing to do with `style=file` option and it does not influence sourced style filename.

Reviewers: rsmith, ioeric, krasimir, sylvestre.ledru, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: MyDeveloperDay, cfe-commits

Patch by:  furdyna

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67750