[LLDB][ClangExpression] Allow expression evaluation from within C++ Lambdas

This patch adds support for evaluating expressions which reference
a captured `this` from within the context of a C++ lambda expression.
Currently LLDB doesn't provide Clang with enough information to
determine that we're inside a lambda expression and are allowed to
access variables on a captured `this`; instead Clang simply fails
to parse the expression.

There are two problems to solve here:
1. Make sure `clang::Sema` doesn't reject the expression due to an
illegal member access.
2. Materialize all the captured variables/member variables required
to evaluate the expression.

To address (1), we currently import the outer structure's AST context
onto `$__lldb_class`, making the `contextClass` and the `NamingClass`
match, a requirement by `clang::Sema::BuildPossibleImplicitMemberExpr`.

To address (2), we inject all captured variables as locals into the
expression source code.

**Testing**

* Added API test