LTO: always parse modules in opaque pointer mode.

Once an LLVMContext has been told it needs to track pointer types, it can no
longer be used to parse opaque modules. However, we are likely  (at least for a
while) to have old LTO .o files in the SDK that need to interoperate with
just-generated ones, so deciding opaqueness based on the first module read
causes linker failures.

This makes the llvm-c LTO interface parse any object it sees in opaque mode,
even if type data is present, which guarantees compatibility.