cmGeneratorTarget: `INTERFACE` targets with C++ modules are also "in"

`INTERFACE` targets with C++ modules are basically BMI-only modules. It
is unknown if they will be useful directly (due to ODR of the `module
M;` initializers needing to live in some specific object file). However,
they will be used to attach BMI-only compilations of `IMPORTED` C++
modules.
1 file changed