LTO API: add lto_module_create_from_memory_with_path.

This function adds an extra path argument to lto_module_create_from_memory.
The path argument will be passed to makeBuffer to make sure the MemoryBuffer
has a name and the created module has a module identifier.

This is mainly for emitting warning messages from the linker. When we emit
warning message on a module, we can use the module identifier.

rdar://15985737