A path of "$(SRCROOT)/../../SomeFile.cpp" would lose the fake C:\ prefix
when the ellipses are collapsed, and trigger a non-absolute File
assertion.

Now that VST3 builds are signed before and after manifest generation, it
may be necessary to force-sign in order to overwrite the
previously-applied signature.

To prevent too much disruption to people who have our public projects as
part of their build or testing systems we are temporarily ignoring a
warning about using Xcode 15.

Co-authored-by: Roland Rabien <figbug@gmail.com>

This reverts commit 82377a787a.

Reintegrates changes from cf9fc84669