[Bug libstdc++/103755] {has,use}_facet() and iostream constructor performance

["dprokoptsev at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103755

--- Comment #6 from Dmitry Prokoptsev <dprokoptsev at gmail dot com> ---
That would also work, I suppose (it even outperforms my original approach by a
tiny bit -- 33 ns for v2 vs 36 for my original implementation).

There are a few build errors in the alternative implementations, however:
is_convertible<locale::facet*, _Facet*>::value is false, as it tries to
downcast the facet -- did you mean is_base_of<>, which would also better align
with the static assert message?

Also, despite the always_inline attribute of __try_use_facet, my attempts to
build the library with -O0 -ggdb and link my benchmark yielded a bunch of
errors like 

    ld:
/home/av/prgs/gcc/debug/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/libstdc++.so:
undefined reference to `std::num_get<char, std::istreambuf_iterator<char,
std::char_traits<char> > > const* std::__try_use_facet<std::num_get<char,
std::istreambuf_iterator<char, std::char_traits<char> > > >(std::locale
const&)'

Suggest explicitly instantiating __try_use_facet where has_facet() and
use_facet() is instantiated.

See attached fix for build problems I discovered.