[lldb/pecoff] Use a different llvm createBinary overload for parsing

Change the code the use the version which accepts a memory buffer,
instead of the one taking a file name.

This ensures we are not loading the file into memory twice
(ObjectFilePECOFF also loads a copy), reducing our memory footprint, as
well as enabling additional goodies in the future, like being able to
open files which don't exist on disk (D83512).