DXC: Annotate new struct fields and enums

[MarijnS95]

For https://github.com/microsoft/win32metadata/pull/474#issuecomment-2322000133.

For DXC_FOURCC it would have been awesome if we could have something like the following, if const methods were supported:

public static const uint DXC_FOURCC(char ch0, char ch1, char ch2, char ch3) => ch0 | (ch1 << 8) | (ch2 << 16) | (ch3 << 24);