VS has different name manglings for classes and structs. With the
new extern(C++, class) and extern(C++, struct) declarations, the
name mangling algorithm is changed to use a mangling different
from the used D entity.
A common use case is to map a value type modeled as class in C++ to
a struct in D.
This is backport of https://github.com/dlang/dmd/pull/5875.
Notably, the glue layer side of the changed multiple interface
inheritance layout (DMD a54e89d) has not been implemented yet.
This corresponds to DMD commit 3f6a763c0589dd03c1c206eafd434b593702564e.
Also adds the CMake infrastructure to compile and link the D source files.
The build is partially broken:
- A few files in Phobos and druntime do not build
- MSVC build is broken because of unresolved symbols involving reals
