[LLD] [COFF] Add an assert regarding the RVA of exported symbols. NFC.

[mstorsjo]

Summary

As this isn't handled as a regular relocation, the normal handling of
maybeReportRelocationToDiscarded in Chunks.cpp doesn't apply here.

This would have caught the issue fixed by
82de4e075339f5ad8d68cfe31eb45b771d4750ae.

[rnk]

lgtm

lld/COFF/DLL.cpp

for (const Export &e : config->exports) {
       uint8_t *p = buf + e.ordinal * 4;
       uint32_t bit = 0;
       // Pointer to thumb code must have the LSB set, so adjust it.
       if (config->machine == ARMNT && !e.data)
         bit = 1;
       if (e.forwardChunk) {
         write32le(p, e.forwardChunk->getRVA() | bit);
       } else {
+        assert(cast<Defined>(e.sym)->getRVA() != 0 &&
+               "Exported symbol unmapped");
         write32le(p, cast<Defined>(e.sym)->getRVA() | bit);
       }
     }
   }
 
 private:
   size_t size;
 };