[cling] Do not suppress all diagnostics during function lookup

[vepadulano]

This is the first part of #12449

I'm still trying to understand how to create a test. I used the original reproducer

template <typename T>
class Helper {

public:

  Helper() {}

  std::size_t operator() () const {
    const std::size_t res = 0;
    res = T{0, 0}.size();
    return res;
  }

};

template <typename H>
std::size_t call_helper(const H &helper) {
  return helper();
}

And ran it with root, but I see the same errors printed both after this changes and with ROOT master

Processing t.C...
In file included from input_line_8:1:
/Users/vpadulan/Programs/rootproject/t.C:10:9: error: cannot assign to variable 'res' with const-qualified type 'const std::size_t' (aka 'const unsigned long')
    res = T{0, 0}.size();
    ~~~ ^
/Users/vpadulan/Programs/rootproject/t.C:18:10: note: in instantiation of member function 'Helper<std::vector<double, std::allocator<double> > >::operator()' requested here
  return helper();
         ^
/Users/vpadulan/Programs/rootproject/t.C:24:5: note: in instantiation of function template specialization 'call_helper<Helper<std::vector<double, std::allocator<double> > > >' requested here
    call_helper(h);
    ^
/Users/vpadulan/Programs/rootproject/t.C:9:23: note: variable 'res' declared const here
    const std::size_t res = 0;
    ~~~~~~~~~~~~~~~~~~^~~~~~~
Assertion failed: (!m_Unloading && "Must not nest within unloading transaction"), function addNestedTransaction, file Transaction.cpp, line 98.

[vepadulano]

This PR is currently blocked by #20952

[github-actions]

Test Results

    22 files      22 suites   3d 23h 39m 25s ⏱️
 3 812 tests  3 809 ✅ 0 💤  3 ❌
76 720 runs  76 701 ✅ 7 💤 12 ❌

For more details on these failures, see this check.

Results for commit 25e1e2f.