[AArch64] Add support for NEON scalar floating-point absolute difference.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195804 91177308-0d34-0410-b5e6-96231b3b80d8
Remove 'DistinctSpellings' support from Attr.td and change its only user to
look at the attribute spelling instead. The 'ownership_*' attributes should
probably be split into separate *Attr classes, but that's more than I wanted to
do here.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195805 91177308-0d34-0410-b5e6-96231b3b80d8