[AArch64] Improve TargetParser API

Re-land with constexpr StringRef::substr():

The TargetParser depends heavily on a collection of macros and enums to tie
together information about architectures, CPUs and extensions. Over time this
has led to some pretty awkward API choices. For example, recently a custom
operator-- has been added to the enum, which effectively turns iteration into
a graph traversal and makes the ordering of the macro calls in the header
significant. More generally there is a lot of string <-> enum conversion
going on. I think this shows the extent to which the current data structures
are constraining us, and the need for a rethink.

Key changes:

 - Get rid of Arch enum, which is used to bind fields together. Instead of
   passing around ArchKind, use the named ArchInfo objects directly or via
   references.

 - The list of all known ArchInfo becomes an array of pointers.

 - ArchKind::operator-- is replaced with ArchInfo::implies(), which defines
   which architectures are predecessors to each other. This allows features
   from predecessor architectures to be added in a more intuitive way.

 - Free functions of the form f(ArchKind) are converted to ArchInfo::f(). Some
   functions become unnecessary and are deleted.

 - Version number and profile are added to the ArchInfo. This makes comparison
   of architectures easier and moves a couple of functions out of clang and
   into AArch64TargetParser.

 - clang::AArch64TargetInfo ArchInfo is initialised to Armv8a not INVALID.

 - AArch64::ArchProfile which is distinct from ARM::ArchProfile

 - Give things sensible names and add some comments.

Differential Revision: https://reviews.llvm.org/D138792