Fix latent bug in .debug_names file-name handling

Fix latent bug in .debug_names file-name handling

An internal Ada test case showed that the .debug_names code does not
compute the same list of file names as the partial symbol reader.  In
particular, the partial symbol reader uses the DW_AT_name of the CU:

  /* Allocate a new partial symbol table structure.  */
  filename = dwarf2_string_attr (comp_unit_die, DW_AT_name, cu);
  if (filename == NULL)
    filename = "";

  pst = create_partial_symtab (per_cu, filename);

This patch changes the .debug_names reader to follow.

gdb/ChangeLog
2019-09-10  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (dw2_get_file_names_reader): Add the
	CU's file name to the results.

gdb/testsuite/ChangeLog
2019-09-10  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/dgopt.exp: New file.
	* gdb.ada/dgopt/x.adb: New file.