Bug 1898651 - Use -Zi instead of -Z7 with ml r=firefox-build-system-reviewers,glandium

The debug flag for ml.exe and ml64.exe is -Zi instead of -Z7,
which is used with clang-cl.

Set this when CC_TYPE is "clang-cl" to match the if-statements in
`default_debug_flags` and `as_info` in toolchain.configure.

Fixes warning A4018: invalid command-line option.

Differential Revision: https://phabricator.services.mozilla.com/D211480