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.
Added support for incremental mode 8 and 28 ie. `frontend::EmitBC:` and `frontend::PrintPreprocessedInput:`
Added supporting clang tests to test in clang-repl mode
Reviewed By: v.g.vassilev
Differential Revision: https://reviews.llvm.org/D125946