[LTO] Fix lto_module_create_in_codegen_context return value on error

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

Reviewed By: pete

Differential Revision: https://reviews.llvm.org/D136769

llvm/tools/lto/lto.cpp

@@ -290,6 +290,8 @@ lto_module_t lto_module_create_in_codegen_context(const void *mem,
       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());
 }