Import memory caused to `Fatal: Module::addMemory: 0 already exists`

[MaxGraey]

After #4811 we find out that BinaryenSetMemory + BinaryenAddMemoryImport produces fatal error:

Fatal: Module::addMemory: 0 already exists

[MaxGraey]

Currently BinaryenSetMemory and BinaryenAddMemoryImport both create new memory and add it via ((Module*)module)->addMemory internally. I propose change BinaryenAddMemoryImport only to modify memory->module and memory->base like:

// Or create new API - BinaryenSetMemoryImport
void BinaryenAddMemoryImport(BinaryenModuleRef module,
                             const char* name,
                             const char* externalModuleName,
                             const char* externalBaseName,
                             /* unused */ uint8_t shared) {
  auto* memory = ((Module*)module)->getMemoryOrNull(name);
  if (memory == nullptr) {
    Fatal() << "invalid memory '" << name 
            << "'. Make sure BinaryenSetMemory was called before BinaryenAddMemoryImport";
  }
  memory->module = externalModuleName;
  memory->base = externalBaseName;
}

instead:

void BinaryenAddMemoryImport(BinaryenModuleRef module,
                             const char* internalName,
                             const char* externalModuleName,
                             const char* externalBaseName,
                             uint8_t shared) {
  auto memory = Builder::makeMemory(internalName);
  memory->module = externalModuleName;
  memory->base = externalBaseName;
  memory->shared = shared;
  ((Module*)module)->addMemory(std::move(memory));
}

WDYT?

[kripken]

Sounds reasonable. But we can make it backwards compatible too, that is, by gracefully handling the case where the memory already exists. I think that would be best.