* dwarf2read.c (struct dwarf2_per_cu_data): New member tu_read.
(fill_in_sig_entry_from_dwo_entry): Reorganize asserts.
Add assert of sig_entry->dwo_unit == NULL.
(lookup_dwo_signatured_type): Don't assign TU to a DWO if the TU
had already been read.
(read_signatured_type): Set per_cu.tu_read.
testsuite/
* gdb.dwarf2/fission-mix.exp: New file.
* gdb.dwarf2/fission-mix.h: New file.
* gdb.dwarf2/fission-mix.c: New file.
* gdb.dwarf2/fission-mix2.c: New file.
