[PECOFF] Remove COFF header from PE header for simplicity.

COFF header is always present both in executable and in object file. PE header
is present only in executable. So the natural way to handle PE/COFF file is
treating COFF is mandatory header and PE is optional. Current data structre
does not allow it, because PE header includes COFF header. Removing COFF
header will simplify the code to handle PE/COFF files.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D952
[PECOFF][Writer] Structure of pe32_header was changed.

Change for http://llvm-reviews.chandlerc.com/D952

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D953