[NFC][XCOFF] Replace structs FileHeader32/SectionHeader32 with constants.

Summary: Some structs like FileHeader32/SectionHeader32
defined in llvm/include/llvm/BinaryFormat/XCOFF.h seem
unnecessary, because we only need their size. So this
patch removes them and defines size constants directly.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D103901