[PECOFF] Remove ResolvableSymbols to simplify.

We had such class there because of InputGraph abstraction.
Previously, no one except InputGraph itself has complete picture of
input file list. In order to create a set of all defined symbols,
we had to use some indirections there to workaround InputGraph.

It can now be rewritten as simple code. No change in functionality.

llvm-svn: 226319