[clang] Fix Lambda Mangling in Namespace-Scope Variable Initializers. (PR #159115)

[Richard Smith via cfe-commits]

zygoloid wrote:

Do we also have test coverage for mangling the names of lambdas that appear in inline variables and variable templates? Those are the cases where it's critical to get this right in plain C++ without language extensions. Also, do we have test coverage for the case where multiple lambdas (with either the same or different lambda-sigs) appear in the same variable?

https://github.com/llvm/llvm-project/pull/159115