Revert commit 5fcadc48 (MSVC: Default to -ZI instead of /Zi for x86
and x64, 2022-05-24).  The `-ZI` flag is incompatible with the `-GL`
flag used for IPO, and so is not an unconditionally better default.
Revert the change pending future design of a first-class setting for
MSVC debug info format that can be automatically reconciled with IPO
settings.

That commit introduced policy CMP0138, but we already have later policy
numbers used too.  Leave placeholder text to avoid policy renumbering.

Issue: #23607, #10189