CheckIPOSupported: Compile check using flags of calling project

Forward CMAKE_<LANG>_FLAGS and CMAKE_<LANG>_FLAGS_DEBUG from the calling project into the test project. The set of flags may affect the availability of IPO support. Since this may change the result of the check for existing projects, add a policy for compatibility.

This was discovered after !7295 (merged) introduced policy CMP0138 to switch our default for MSVC's debug info flag. The -ZI flag is incompatible with the -GL flag used for IPO, so CMP0138 was reverted by !7344 (merged) pending future work on an alternative solution. Re-use the CMP0138 policy number for this change to CheckIPOSupported instead.

Fixes: #23607 (closed)
Backport: release