Adds a Pragma spelling for attributes to tablegen and makes use of it for loop
hint attributes. Includes tests for pragma printing and for attribute order
which is incorrectly reversed by ParsedAttributes.

Reviewed by Aaron Ballman


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210925 91177308-0d34-0410-b5e6-96231b3b80d8