[clang] [clang] Handle constructor closures with consteval default args (PR #203554)

[Hans Wennborg via cfe-commits]

zmodem wrote:

This attaches CXXDefaultArgExpr's to the CXXConstructorDecl's conceptually (as discussed on the bug), by means of a side table in ASTContext (inspired by DeclAttrs which similarly don't apply to most nodes). In a way this is similar to the old solution in dfa6d2067c8c782c83be69a61fe59a0240535df6 but this also supports serialization. Please take a look!

https://github.com/llvm/llvm-project/pull/203554