[libc++][NFC] Mark values in gdb pretty print comparison functions as live to prevent...

[libc++][NFC] Mark values in gdb pretty print comparison functions as live to prevent values being optimized out.

It appears when testing LLVM 13 on Power, we run into failures with the
`libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp` test case optimizing
values out.

Despite some the functions in the test already being marked with optnone,
adding the `MarkAsLive()` calls inside of the pretty printer comparison functions
resolves the issues of the values being optimized out.

This patch aims to address https://llvm.org/PR51675.

Differential Revision: https://reviews.llvm.org/D109204