[clang] [clang] Don't use `VarDecl` of local variables as `ManglingContextDecl` for lambdas (PR #179035)

[Jan Kokemüller via cfe-commits]

jiixyj wrote:

`ManglingContextDecl` is also used in `SemaExpr.cpp:HandleImmediateInvocations` for some unrelated things, so it's not that simple to change.

But I found another simpler approach: I could remove `ContextKind::NonInlineInModulePurview` and just do a simple check for the modules special case with a helper function. All in the `ContextKind::Normal` case.

https://github.com/llvm/llvm-project/pull/179035