[clang] [Clang] Skip past code generation for unevaluated lambdas (PR #124572)

[via cfe-commits]

cor3ntin wrote:

Fundamentally the issue is that we emit codegen decl for all function instantiations (and presumably that gets ignored for decls that are not ODR-used).
But in the process, we try to produce a mangling, which we cannot do for some unevaluated lambdas whose mangling depends on the outerscope that is not yet instantiated.

I'm not sure this is the best fix but neither @zyn0217 nor myself can come up with a better idea.

https://github.com/llvm/llvm-project/pull/124572