travis.yml: Use clang++ in the Clang tests

travis.yml: Use clang++ in the Clang tests

Our configure script does not look for clang++ automatically, so we
should use --cxx=clang++ to make sure that we test our C++ code with
Clang, too. And while we're at it, also use --host-cc=clang here
to avoid that we use the normal "cc" as host C compiler.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200518083316.25065-1-thuth@redhat.com>
Message-Id: <20200520140541.30256-4-alex.bennee@linaro.org>