[gdb/symtab] Handle named DW_TAG_unspecified_type DIE

[gdb/symtab] Handle named DW_TAG_unspecified_type DIE

With the test-case included in the patch, we run into:
...
(gdb) info types -q std::nullptr_t^M
During symbol reading: unsupported tag: 'DW_TAG_unspecified_type'^M
^M
File /usr/include/c++/7/x86_64-suse-linux/bits/c++config.h:^M
2198:   typedef decltype(nullptr) std::nullptr_t;^M
(gdb) FAIL: gdb.dwarf2/nullptr_t.exp: info types -q std::nullptr_t \
  without complaint
...

Fix the complaint by handling DW_TAG_unspecified_type in new_symbol, and verify
in the test-case using "maint print symbols" that the symbol exists.

Tested on x86_64-linux, with gcc 7.5.0 and clang 13.0.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17271