Propagate `cfg` annotations on COM class methods

[rylev]

It can be useful for COM class methods to be conditionally compiled. We should propagate conditional compilation.

This would be useful for use in hassle-rs.

[MarijnS95]

Hoping we don't need it "soon" when I get around to fixing vtable misalignment issues in DirectXShaderCompiler. Might still be useful for us in the short term and other projects later on, though 🙂