[io] read versioned class from file even if in-memory-class is unversioned

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://its.cern.ch/jira/browse/ROOT-5306

Reproducer:

root
.L /tmp/MySubClass.cxx+
TFile f("/tmp/mysub.root", "RECREATE");
MySubClass msc;
msc.id = 33;
f.WriteObjectAny(&msc, "MySubClass", "msc");
f.Close();
.q
root
.L /tmp/MySubClassUnv.cxx+
auto file = TFile::Open("/tmp/mysub.root", "READ");
auto msc = file->Get<MySubClass>("msc")
(msc->id == 33)
.q

with MySubClass.cxx

class MySubClass { public: int id; ClassDef(MySubClass, 3) };

and MySubClassUnv.cxx

class MySubClass { public: int id; };

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    18 files      18 suites   4d 2h 4m 6s ⏱️
 2 739 tests  2 728 ✅ 0 💤 11 ❌
47 931 runs  47 920 ✅ 0 💤 11 ❌

For more details on these failures, see this check.

Results for commit 1aada00.

♻️ This comment has been updated with latest results.

[ferdymercury]

@pcanal
gentle reminder to 'squash on merge' (if you approve).