This removes a workaround for a libstdc++ bug where we need to eagerly
parse the exception specification.
libstdc++ had a bug where it shadows std::swap with a
member swap function then tries to call std::swap unqualified from the
exception specification of that function.
We would workaround that by detecting whether we were in such a case
and turn off delay-parsing of exception specifications.
Libstdc++ 6.1 (released 2016-04-27) appears to have resolved it as side-effect
of commit ddb63209a8d (2015-06-05).
GCC 10.1, released on May 7, 2020, appears to parse the exception
specification correctly, and does not include any sort of workaround for
std:: / system headers.
Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>