[COFF] Respect weak externals for mangled symbol searching

We were previously ignoring weak externals during these searches (which
are used for the entry point, exports, and subsystem inference), which
differed from link.exe behavior. It also meant that we could get
different behavior when linking an object file directly vs. packaging it
into a static library, because static library symbol name directories
include weak externals.

Reviewed By: mstorsjo, yozhu

Differential Revision: https://reviews.llvm.org/D139764