[clang] fix crash on generic lambda with lambda in decltype

Relevant issue: https://github.com/llvm/llvm-project/issues/59771

During the instantiation of a generic lambda, a non-generic lambda in
the trailing `decltype` is a `DeclContext` but not a dependent context,
so we shouldn't call `PerformDependentDiagnostics` on it.

Differential Revision: https://reviews.llvm.org/D140838