Support ObjC in IncludeInserter

Update IncludeSorter/IncludeInserter to support objective-c google style (part 1):

1) Correctly consider .mm/.m extensions
2) Correctly categorize category headers.
3) Add support for generated files to go in a separate section of imports

Reviewed By: alexfh, gribozavr2

Patch by Joe Turner.

Differential Revision: https://reviews.llvm.org/D89276