[LTO] Fix lto_module_create_in_codegen_context return value on error

[steven_wu]

Summary

According to the documentation, lto_module_create_in_codegen_context
should return NULL on error but currently it is accidentally return
error_code. Since this is a bug fix and it seems to be a one-off bug
that only affects this API, there is no need to bump API version.

rdar://101505192

[pete]

Matches the other lto methods, which do the same error check. LGTM!

llvm/tools/lto/lto.cpp

lto_module_t lto_module_create_in_codegen_context(const void *mem,
                                                   size_t length,
                                                   const char *path,
                                                   lto_code_gen_t cg) {
   lto_initialize();
   llvm::TargetOptions Options =
       codegen::InitTargetOptionsFromCodeGenFlags(Triple());
   ErrorOr<std::unique_ptr<LTOModule>> M = LTOModule::createFromBuffer(
       unwrap(cg)->getContext(), mem, length, Options, StringRef(path));
+  if (!M)
+    return nullptr;
   return wrap(M->release());
 }
 
 void lto_module_dispose(lto_module_t mod) { delete unwrap(mod); }
 
 const char* lto_module_get_target_triple(lto_module_t mod) {
   return unwrap(mod)->getTargetTriple().c_str();
 }