COFF: Load inputs immediately instead of adding them to a queue.

This patch replaces the symbol table's object and archive queues, as well as
the convergent loop in the linker driver, with a design more similar to the
ELF linker where symbol resolution directly causes input files to be added to
the link, including input files arising from linker directives. Effectively
this removes the last vestiges of the old parallel input file loader.

Differential Revision: https://reviews.llvm.org/D27660

llvm-svn: 289409