[clang-format] Modified SortIncludes and IncludeCategories to priority for...

[clang-format] Modified SortIncludes and IncludeCategories to priority for sorting #includes within the Group Category.

Summary:
This new Style rule is made as a part of adding support for NetBSD KNF in clang-format. NetBSD have it's own priority of includes which should be followed while formatting NetBSD code. This style sorts the Cpp Includes according to the priorities of NetBSD, as mentioned in the [Style Guide](http://cvsweb.netbsd.org/bsdweb.cgi/src/share/misc/style?rev=HEAD&content-type=text/x-cvsweb-markup)
 The working of this Style rule shown below:

**Configuration:**
This revision introduces a new field under IncludeCategories named `SortPriority` which defines the priority of ordering the `#includes` and the `Priority` will define the categories for grouping the `#include blocks`.

Reviewers: cfe-commits, mgorny, christos, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: lebedev.ri, rdwampler, christos, mgorny, krytarowski

Patch By: Manikishan

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D64695

llvm-svn: 372919