[cling] Also fwd decl underlying type of using decls: [v6.24]

[Axel-Naumann]

Before, only the using decl itself was forward declared, causing
undeclared identifiers in forward declaration code, as witnessed in
#8499

Given the similarity of using and typedef, merge both into a single
function, making sure both have the same featureset, and through that
fixing this issue as a side-effect.

(cherry picked from commit d026b49)

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-ubuntu16/nortcxxmod, mac1014/python3, mac11.0/cxx17, windows10/cxx14
How to customize builds

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/default.
Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.JupyROOT.roottest_python_JupyROOT_importROOT_notebook
• projectroot.roottest.python.JupyROOT.roottest_python_JupyROOT_simpleCppMagic_notebook
• projectroot.roottest.python.JupyROOT.roottest_python_JupyROOT_thread_local_notebook
• projectroot.roottest.python.JupyROOT.roottest_python_JupyROOT_ROOT_kernel_notebook
• projectroot.roottest.python.JupyROOT.roottest_python_JupyROOT_tpython_notebook

[Axel-Naumann]

C8 failures are expected / unrelated.

[Axel-Naumann]

Confirmed as fixing the underlying issue by Peter Hristov / ALICE. Thanks!

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-ubuntu16/nortcxxmod, mac1014/python3, mac11.0/cxx17, windows10/cxx14
How to customize builds

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/default.
Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See console output.

Errors:

• [2021-06-24T12:39:51.383Z] CMake Error at /data/sftnight/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):

[phsft-bot]

Build failed on ROOT-ubuntu16/nortcxxmod.
Running on sft-ubuntu-1604-2.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2021-06-24T12:40:59.899Z] CMake Error at /mnt/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft10.dyndns.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2021-06-24T12:42:12.164Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):

[phsft-bot]

Build failed on mac1014/python3.
Running on macphsft17.dyndns.cern.ch:/build/jenkins/workspace/root-pullrequests-build
See console output.

Errors:

• [2021-06-24T13:02:11.598Z] CMake Error at /build/jenkins/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):

[phsft-bot]

Build failed on mac11.0/cxx17.
Running on macphsft23.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2021-06-24T13:09:05.538Z] CMake Error at /Users/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):

[phsft-bot]

Build failed on windows10/cxx14.
Running on null:C:\build\workspace\root-pullrequests-build
See console output.

Errors:

• [2021-06-24T14:31:28.577Z] CMake Error at C:/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1043 (message):