[clang] Add test for CWG1837

[[https://wg21.link/p1787 | P1787]]: CWG1837 is resolved by restricting `this` to referring to the innermost enclosing class.
Wording: see changes to [expr.prim.this] and [expr.prim.lambda].

Reviewed By: #clang-language-wg, erichkeane

Differential Revision: https://reviews.llvm.org/D148035