Diagnostics: "Fix" policy warnings in modules

Introduce a mechanism (via the private and undocumented `ISSUE_WARNING`
option to the `cmake_policy` command) to allow built-in modules to issue
policy warnings. Update modules to use this.

This allows modules to issue policy warnings with the `CMD_POLICY`
diagnostic type, which is consistent with internally issued diagnostics.
This also tends to simplify the logic for issuing such warnings in much
the way the helpers introduced in the previous commit did for the C++
code.