[clang] [RFC][C++20][Modules] Fix crash when function and lambda inside loaded from different modules (PR #104512)

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

> I feel the change is somewhat odd to me. Since the description is about the lambda but the change is about VarDecls. I feel there is a mismatch and I feel this may not solve the underlying problem fundamentally. I feel it'll be better to make it more straight forward. e.g., load the context when loading the lambda (or only make it if there is captures.)

It was my initial approach to make sure that clang loads function and its lambdas from the same module. But I haven't found examples in clang how to make that canonical decls for two related things are always loaded from the same module. Clang starts loading function `tryTo` from `thrift_cpp2_base.h` module it follow links and via `declval` function specialization it discovers the lambda from `folly-conv.h` module that becomes canonical so by the time when it recursively returns to the lambda inside `thrift_cpp2_base.h` it just merged it to already known decl from `folly-conv.h`. I don't know how to make sure that deserialization won't intermix. As far as I understand code, clang solves this issue by merging identical decls so I implemented merging for VarDecls that cause this issue. But if there is a way to synchronize loading for a function and it's lambda, it should be more reliable but I failed to find the way. Help with solving this issue is very appreciated, clang seems to have other issues with merging lambdas too but I was not able to create small reproducers in other cases yet.

https://github.com/llvm/llvm-project/pull/104512