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 commit 5fcadc48 (MSVC: Default to -ZI
instead of /Zi for x86 and x64, 2022-05-24) 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
pending future work on an alternative solution.  Re-use the CMP0138
policy number for this change to CheckIPOSupported instead.

Fixes: #23607