[LLD][COFF] Allow overwriting directives exports with cmd-line exports

MSVC link.exe allows overriding exports on the cmd-line with exports seen in OBJ directives. The typical case is what is described in #62329.

Before this patch, trying to override an export with `/export` or `/def` would generate a duplicate warning. This patches tries to replicate the MSVC behavior. A second override on the cmd-line would still generate the warning.

There's still a case which we don't cover: MSVC link.exe is able to demangle an exported OBJ directive function, and match it with a unmangled export function in a .def file. In the meanwhile, one can use the mangled export in the .def to cover that case.

This fixes #62329

Differential revision: https://reviews.llvm.org/D149611