[clang][c++20] Fix false warning for unused private fields when a class has only...

[clang][c++20] Fix false warning for unused private fields when a class has only defaulted comparison operators.

Fixes bug 50263

When "unused-private-field" flag is on if you have a struct with private
members and only defaulted comparison operators clang will warn about
unused private fields.

If you where to write the comparison operators by hand no warning is
produced.

This is a bug since defaulting a comparison operator uses all private
members .

The fix is simple, in CheckExplicitlyDefaultedFunction just clear the
list of unused private fields if the defaulted function is a comparison
function.

Differential revision: https://reviews.llvm.org/D102186