[AArch64][AsmParser] Arch directives should set implied features.

When assembling for example an SVE instruction with the `.arch +sve2` directive,
+sve should be implied by setting +sve2, similar to what would happen if
one would pass the mattr=+sve2 flag on the command-line.

The AsmParser doesn't set the implied features, meaning that the SVE
instruction does not assemble. This patch fixes that.

Note that the same does not hold when disabling a feature. For example,
+nosve2 does not imply +nosve.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D120259