Run IWYU across the entire compiler code base.

[fruffy]

Clean up the compiler includes by running IWYU across them. This also exposes how some files have a massive amount of includes, some of which may not be necessary.

If requested, I can also add a comment per include which lists why that particular include was added.

Also reduces the size of binaries by a little. For example p4c-bm2-ss 201MB -> 198MB.