[clang] Fix linkage of nested lambdas.

patch from Philippe Daouadi <blastrock@free.fr>

This is an attempt to fix
[PR#44368](https://bugs.llvm.org/show_bug.cgi?id=44368)

This effectively reverts [D1783](https://reviews.llvm.org/D1783). It
doesn't break the current tests and fixes the test that this commit
adds.

We now decide of a lambda linkage only depending on the visibility of
its parent context.

Differential Revision: https://reviews.llvm.org/D73701