[LLD] Remove global state in lld/COFF

This patch removes globals from the lldCOFF library, by moving globals
into a context class (COFFLinkingContext) and passing it around wherever
it's needed.

See https://lists.llvm.org/pipermail/llvm-dev/2021-June/151184.html for
context about removing globals from LLD.

I also haven't moved the `driver` or `config` variables yet.

Differential Revision: https://reviews.llvm.org/D109634