[COFF] clean up global resources after completion

Patch by Andrew Kelley.

Previously, running lld::coff::link() twice in the same process would
access stale pointers because of these global variables not being reset.
After this patch, lld::coff::link() can be called any number of times,
just like its ELF and MACH-O counterparts.

Differential Revision: https://reviews.llvm.org/D49856

llvm-svn: 338042