[clangd] ExtractVariable support for C and Objective-C

- Use the expression's type for non-C++ as the variable type. This works
  well, but might not preserve the typedefs due to type
  canonicalization.

- Improve support for Objective-C property references which are
  represented using `ObjCPropertyRefExpr` and `BuiltinType::PseudoObject`.

Differential Revision: https://reviews.llvm.org/D124486