[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