[clang-repl] Add a interpreter-specific overload of operator new for C++ (#76218)

This patch brings back the basic support for C by inserting the required
for value printing runtime only when we are in C++ mode. Additionally,
it defines a new overload of operator placement new because we can't
really forward declare it in a library-agnostic way.

Fixes the issue described in llvm/llvm-project#69072.