PR symtab/15691

	PR symtab/15691
	* 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.