[Sema] Stop stripping CV quals from *this captures in lambdas

It appears we've been incorrectly stripping CV qualifiers when capturing `this` by value inside lambdas.
This patch simply removes the CV stripping code as discussed.

Closes https://github.com/llvm/llvm-project/issues/50866

Reviewed By: #clang-language-wg, cor3ntin, aaron.ballman

Differential Revision: https://reviews.llvm.org/D146168