Refactored the way attribute category headers are handled so that it is possible...

Refactored the way attribute category headers are handled so that it is possible to use custom categories. This allows for moving the consumable attributes (consumable, callable_when, return_typestate, etc) to be grouped together, with a content heading, like they were in the language extensions documentation. Moved the consumable attribute documentation from the language extensions into the attribute documentation table.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201732 91177308-0d34-0410-b5e6-96231b3b80d8