[NFC][COFF] Use COFFSection.MCSection when writeSection

Each COFFSection bind MCSection when created. No need to iterate
throught MCAssembler when writeSection.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D151793

GitOrigin-RevId: 01cc38843d7629a50a8962573116766ad9f9eeaf
1 file changed