[COFF] don't mark lazy symbols as used in regular objects

Summary:
r338767 updated the COFF and wasm linker SymbolTable code to be
strutured more like the ELF linker's. That inadvertedly changed the
behavior of the COFF linker so that lazy symbols would be marked as
used in regular objects. This change adds an overload of the insert()
function, similar to the ELF linker, which does not perform that
marking.

Reviewers: ruiu, rnk, hans

Subscribers: aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51720

llvm-svn: 341585