Analyze include order on a per-file basis.

The include order check would get notified of all include
directives in a depth-first manner.  This created the
possibility of an include directive from a header file
interfering with the sort order of a set of two distinct
blocks from the top level cpp file, if that include directive
was on just the right line.

With this patch we bucket the include directives by the file
in which they appear in and process one bucket at a time,
so that directives from different files do not get mixed
together into the same list.

Reviewed By: alexfh
Differential Revision: https://reviews.llvm.org/D23434

llvm-svn: 278546