gdb/dwarf2: cleanup includes

gdb/dwarf2: cleanup includes

Cleanup includes in dwarf2/*.

 1. Add the necessary includes so that clangd reports no errors when
    opening header files.  This ensures that header files include what
    they use.

 2. Remove all includes reported as unused by clangd (except
    gdb-safe-ctype.h, which I think does some magic that affects what
    follows).

Built-tested --enable-threading at "yes" and "no", since there are some
portions of code gated by `#ifdef CXX_STD_THREAD`.

Change-Id: I21debffcd7c2caf90f08e1e0fbba3ce30422d042
Approved-By: Tom Tromey <tom@tromey.com>