[lld/mac] Don't assert when using -exported_symbol with private symbol

When I added this assert in D93609, it asserted that a symbol that
is privateExtern is also isExternal().

In D98381 the privateExtern check moved into shouldExportSymbol()
but the assert didn't -- now it checked that _every_ non-exported
symbol is isExternal(), which isn't true. Move the assert into the
privateExtern check where it used to be.

Fixes PR50098.

Differential Revision: https://reviews.llvm.org/D101223