[clang] Extend pragma dump to support expressions

Extend `#pragma clang __debug dump` to support not only single identifier, but an expression as well. This makes it possible to test ADL and overload resolution directly, without being creative to make them observable via diagnostics (e.g. when [[ http://eel.is/c++draft/over.match.best | over.match.best ]] is involved). This implementation has a known limitation of not supporting dependent expressions properly, but it's quite useful even without such support.

Differential Revision: https://reviews.llvm.org/D144115