[ELF] - Recommit r298078 "[ELF] - Simplify logic of creating "COMMON" section."

With fix of next warning:
Writer.cpp:361:3: warning: suggest parentheses around assignment used as truth value [-Wparentheses]

Original commit message:

Patch reuses BssSection section to simplify creation of 
COMMON section.

Differential revision: https://reviews.llvm.org/D30690

llvm-svn: 298086