[AST] Mangle LambdaContextDecl for top level decl

Summary:

Bug filed here: https://bugs.llvm.org/show_bug.cgi?id=45213

To resolve it, we let the checks for mangling LambdaContextDecl to be analogous to ItaniumMangle strategy: https://github.com/llvm/llvm-project/blob/master/clang/lib/AST/ItaniumMangle.cpp#L1829

Differential Revision: https://reviews.llvm.org/D80153