[clang-repl] Introduce Value to capture expression results

This is the second part of the below RFC:
https://discourse.llvm.org/t/rfc-handle-execution-results-in-clang-repl/68493



This patch implements a Value class that can be used to carry expression
results in clang-repl. In other words, when we see a top expression
without semi, it will be captured and stored to a Value object. You can
explicitly specify where you want to store the object, like:

```
Value V;
llvm::cantFail(Interp->ParseAndExecute("int x = 42;"));
llvm::cantFail(Interp->ParseAndExecute("x", &V));
```

`V` now stores some useful infomation about `x`, you can get its real
value (42), it's `clang::QualType` or anything interesting.

However, if you don't specify the optional argument, it will be captured
to a local variable, and automatically called `Value::dump`, which is
not implemented yet in this patch.

Signed-off-by: Jun Zhang <jun@junz.org>