[clang][AArch64] readd support for 'p' inline asm constraint

Summary:
Was accidentally removed by commit af64948e when it overrode
TargetInfo::convertConstraint.

Fixes: pr/45225

Reviewers: eli.friedman, sdesmalen

Reviewed By: sdesmalen

Subscribers: echristo, sdesmalen, kristof.beyls, cfe-commits, kmclaughlin, srhines

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76297