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 5fcadc481e (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
Add IPO support for Visual Studio (which is referred to by VS as
"Link Time Code Generation" and "Whole Program Optimization"), for
VS version >= 10. This allows CMake/VS users to enable IPO by setting
property `INTERPROCEDURAL_OPTIMIZATION`.
Fixes: #16748
Previously the `INTERPROCEDURAL_OPTIMIZATION` target property was
honored only for the Intel compiler on Linux and otherwise ignored. In
order to add support for more compilers incrementally without changing
behavior in the future, add a new policy whose NEW behavior enforces the
`INTERPROCEDURAL_OPTIMIZATION` property. Add flags for supported
compilers and otherwise produce an error.
