Sort the enums in Attributes.h in case insensitive alphabetical order.

Sort the enums in Attributes.h in case insensitive alphabetical order.

Sort the enums in preparation for moving the attributes to a table-gen
file.

rdar://problem/19836465

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252692 91177308-0d34-0410-b5e6-96231b3b80d8