[RuntimeDyld] Don't error on symbols that resolve to 0

[MoritzS]

Summary

A symbol can have 0 as its valid address. This happens with weak
undefined symbols in ELF, for example.

[MoritzS]

This is now solved in D97898.

llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp

if (Name.size() == 0) {
         // We found the symbol in our global table.  It was probably in a
         // Module that we loaded previously.
         const auto &SymInfo = Loc->second;
         Addr = getSectionLoadAddress(SymInfo.getSectionID()) +
                SymInfo.getOffset();
         Flags = SymInfo.getFlags();
       }
 
-      // FIXME: Implement error handling that doesn't kill the host program!
-      if (!Addr)
-        report_fatal_error("Program used external function '" + Name +
-                           "' which could not be resolved!");
-
       // If Resolver returned UINT64_MAX, the client wants to handle this symbol
       // manually and we shouldn't resolve its relocations.
       if (Addr != UINT64_MAX) {
 
         // Tweak the address based on the symbol flags if necessary.
         // For example, this is used by RuntimeDyldMachOARM to toggle the low bit
         // if the target symbol is Thumb.
         Addr = modifyAddressBasedOnFlags(Addr, Flags);