[PECOFF] Refactoring: Split FileCOFF c'tor. No functionality change.

Split FileCOFF's constructor into mainly two private methods.
One method is responsible to iterate over symbol tables, and other
method is to atomize defined atoms. This is for readability and
no changes in functionality.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D940

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183708 91177308-0d34-0410-b5e6-96231b3b80d8