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

llvm-svn: 315931