[gdb/symtab] Handle DW_TAG_type_unit in process_psymtab_comp_unit

[gdb/symtab] Handle DW_TAG_type_unit in process_psymtab_comp_unit

When running test-case gdb.cp/cpexprs-debug-types.exp with target board
unix/gdb:debug_flags=-gdwarf-5, I run into:
...
(gdb) file cpexprs-debug-types^M
Reading symbols from cpexprs-debug-types...^M
ERROR: Couldn't load cpexprs-debug-types into GDB (eof).
ERROR: Couldn't send delete breakpoints to GDB.
ERROR: GDB process no longer exists
GDB process exited with wait status 23054 exp9 0 0 CHILDKILLED SIGABRT SIGABRT
...

We're running into this abort in process_psymtab_comp_unit:
...
  switch (reader.comp_unit_die->tag)
    {
    case DW_TAG_compile_unit:
      this_cu->unit_type = DW_UT_compile;
      break;
    case DW_TAG_partial_unit:
      this_cu->unit_type = DW_UT_partial;
      break;
    default:
      abort ();
    }
...
because reader.comp_unit_die->tag == DW_TAG_type_unit.

Fix this by adding a DW_TAG_type_unit case.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR symtab/27333
	* dwarf2/read.c (process_psymtab_comp_unit): Handle DW_TAG_type_unit.