Sort Attributes by "HeaderName"

Attributes in the docs were previously sorted (apparently)
by the attribute name, so AnyX86Interrupt ended up being the
first one, rather than in a meaningful place. This resulted in the
4 'interrupt' titled sections being all in different places.

This replaces it with a naive alphabetical sort (case sensitive, underscore
and special characters first, etc).

Differential Revision: https://reviews.llvm.org/D38969


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315931 91177308-0d34-0410-b5e6-96231b3b80d8