[PECOFF] Simplicy FileCOFF ctor. No functionality change.

Member functions to read the symbol table had too many parameters to propagate
all the temporary information from one to another. By storing the information
to data members, we can simplify the function signatures and improve the
readability.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187321 91177308-0d34-0410-b5e6-96231b3b80d8