[AArch64] [SEH] Rename pac_sign_return_address to pac_sign_lr

This new opcode was initially documented as "pac_sign_return_address"
in https://github.com/MicrosoftDocs/cpp-docs/pull/4202, but was
soon afterwards renamed into "pac_sign_lr" in
https://github.com/MicrosoftDocs/cpp-docs/pull/4209, as the other
name was unwieldy, and there were no other external references to
that name anywhere.

Rename our external .seh assembler directive - it hasn't been merged
for very long yet, so there's probably no external use to account for.

Rename all other internal references to the opcode similarly.

Differential Revision: https://reviews.llvm.org/D135762