[Clang][Sema] Properly get captured 'this' pointer in lambdas with an explicit...

[Clang][Sema] Properly get captured 'this' pointer in lambdas with an explicit object parameter in constant evaluator (#81102)

There were some bugs wrt explicit object parameters in lambdas in the
constant evaluator:
- The code evaluating a `CXXThisExpr` wasn’t checking for explicit
object parameters at all and thus assumed that there was no `this` in
the current context because the lambda didn’t have one, even though we
were in a member function and had captured its `this`.
- The code retrieving captures as lvalues *did* account for explicit
object parameters, but it did not handle the case of the explicit object
parameter being passed by value rather than by reference.

This fixes #80997.

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Co-authored-by: cor3ntin <corentinjabot@gmail.com>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>