[clang] Make sure all C++ DR tests are running with `-pedantic-errors` (#94203)

In #94167 I found out that `cwg28xx.cpp` has been running without
`-pedantic-errors` and fixed that. This patch fixes that for the rest of
the test suite. Only one test was affected with a trivial fix (warning
was escalated to an error).

I'm intentionally leaving out test for CWG2390, because it requires
major surgery. It's addressed in #94206.