Investigate how to fix usage of Sccache for Windows Ninja Multi Config MSVC builds

As per https://codereview.qt-project.org/c/qt/qt5/+/232218/62/coin/platform_configs/cmake_platforms.yaml#25

and
https://github.com/mozilla/sccache/issues/242

it appears sccache can't deal with MSVC debug builds that put the debug information into a separate pdb file.

This prevents us from enabling Sccache for caching build artifacts for Windows.

We should investigate if it's possible to fix ccache upstream, or there is some workaround we can use.

One proposed workaround is to switch from /Zi to /Z7 to keep the debug info in object files, and then separate it out into pdb files, similar to how it's done for macOS / Linux with --separate-debug-info.

Reference flags: https://docs.microsoft.com/en-us/cpp/build/reference/z7-zi-zi-debug-information-format?view=vs-2019