Replace our hard-coded default for `/RTC1` with a first-class
abstraction to select runtime checks from an enumeration of logical
names. Add a `MSVC_RUNTIME_CHECKS` target property and corresponding
`CMAKE_MSVC_RUNTIME_CHECKS` variable.
Removing the old default flag requires a policy because existing
projects may rely on string processing to edit them and choose
runtime checks under the old behavior. Add policy CMP0184 to
provide compatibility.
Fixes: #26614
2b2344b412