[NFC][AArch64] Use optional returns in target parser instead of 'invalid' objects

[NFC][AArch64] Use optional returns in target parser instead of 'invalid' objects

This updates the parsing methods in AArch64's Target Parser to make use
of optional returns instead of "invalid" enum values, making the API's
behaviour clearer.

Reviewed By: lenary, tmatheson

Differential Revision: https://reviews.llvm.org/D142539