[lld-macho] Replace debug-info-related assert with FIXME

[int3]

Summary

We'll need to properly handle object files with multiple source inputs
eventually, but remove the assert for now so we can successfully emit binaries
for testing.

[smeenai]

LGTM

lld/MachO/InputFiles.cpp

auto *ctx = make<DWARFContext>(
       },
       [&](Error warning) {
         warn(toString(this) + ": " + toString(std::move(warning)));
       });
 
   // TODO: Since object files can contain a lot of DWARF info, we should verify
   // that we are parsing just the info we need
   const DWARFContext::compile_unit_range &units = ctx->compile_units();
+  // FIXME: There can be more than one compile unit per object file. See
+  // PR48637.
   auto it = units.begin();
   compileUnit = it->get();
-  assert(std::next(it) == units.end());
 }
 
 // The path can point to either a dylib or a .tbd file.
 static Optional<DylibFile *> loadDylib(StringRef path, DylibFile *umbrella) {
   Optional<MemoryBufferRef> mbref = readFile(path);
   if (!mbref) {
     error("could not read dylib file at " + path);
     return {};