* dwarf2read.c (read_structure_type): Delete outdated comments.
Make more robust when run in parallel mode.

Since we're not compiling with gcc, we don't know where the
DWO file will ultimately be built.  It could be built in
testsuite/gdb.dwarf2/foo (non-parallel mode) or
testsuite/outputs/gdb.dwarf2/foo (parallel mode).

	* gdb.dwarf2/fission-reread.S: Remove directory from .dwo file path.
	* gdb.dwarf2/fission-reread.exp: Set debug-file-directory before
	loading file.  Add test for TU lookup.