[PECOFF] Add support for bigobj

lld shouldn't directly use the COFF header nor should it use raw
coff_symbols.  Instead, query the header properties from the
COFFObjectFile and use COFFSymbolRef to abstractly reference COFF
symbols.

This is just enough to get lld compiling with the changes to
llvm::object.  Bigobj specific testing will come later.

Differential Revision: http://reviews.llvm.org/D5280

llvm-svn: 217497