[yaml2obj][MachO] Don't fill dummy data for virtual sections

Summary:
Currently, MachOWriter::writeSectionData writes dummy data (0xdeadbeef) to fill section data areas in the file even if the section is a virtual one. Since virtual sections don't occupy any space in the file, writing dummy data could results the  "OS.tell() - fileStart <= Sec.offset" assertion failure.

This patch fixes the bug by simply not writing any dummy data for virtual sections.

Reviewers: beanz, jhenderson, rupprecht, alexshap

Reviewed By: alexshap

Subscribers: compnerd, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62991

llvm-svn: 363525