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
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3db1603..1139116 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -292,6 +292,24 @@
 - clang-format gets a new option called ``--dry-run`` or ``-n`` to emit a
   warning.
 
+- Option *IncludeIsMainSourceRegex* added to allow for additional
+  suffixes and file extensions to be considered as a source file
+  for execution of logic that looks for "main *include* file" to put
+  it on top.
+
+  By default, clang-format considers *source* files as "main" only when
+  they end with: ``.c``, ``.cc``, ``.cpp``, ``.c++``, ``.cxx``,
+  ``.m`` or ``.mm`` extensions. This config option allows to
+  extend this set of source files considered as "main".
+            
+  For example, if this option is configured to ``(Impl\.hpp)$``,
+  then a file ``ClassImpl.hpp`` is considered "main" (in addition to
+  ``Class.c``, ``Class.cc``, ``Class.cpp`` and so on) and "main
+  include file" logic will be executed (with *IncludeIsMainRegex* setting
+  also being respected in later phase). Without this option set,
+  ``ClassImpl.hpp`` would not have the main include file put on top
+  before any other include.
+
 libclang
 --------