CXXTarget does not have _create_target

[ryanmrichard]

Describe the bug
Classes which derive from CXXTarget are expected to override a virtual function _create_target; however, CXXTarget does not actually declare a virtual _create_target. The code still works as is because CMakePPLang classes behave like Python classes, i.e., method resolution is done at runtime and can see all methods available to the object, including those from the derived classes.

We may want to take a page from C++'s playbook and have developers mark functions intended to override functions in some manner. Perhaps instead of:

cpp_member(_create_target CXXExecutable)
function("${_create_target}" self)
# implementation
endfunction()

it becomes:

cpp_override(_create_target CXXExecutable)
function("${_create_target}" self)
# implementation
endfunction()

where cpp_override wraps cpp_member, but additionally checks that one of the base classes declares a virtual function with the same signature (may need some type gymnastics for the self object).

To Reproduce
Look at the CXXTarget class's source code

Expected behavior
CXXTarget should declare the virtual function.

Additional context
N/A