[ARM][AArch64] Use StringRef in TargetParser structs

The invalid case is now represented by an empty StringRef rather than
a nullptr.

Previously ARCH_FEATURE was build from SUB_ARCH by prepending "+".
This is now reverse, so that the "+arch-feature" is now visible in
the .def, which is a bit clearer. This meant converting one StringSwitch
into a loop.

Removed getters which are now mostly unnecessary.

Removed some old FIXMEs.

Differential Revision: https://reviews.llvm.org/D138026