[clang] Add [is|set]Nested methods to NamespaceDecl

Adds support for NamespaceDecl to inform if its part of a nested namespace.
This flag only corresponds to the inner namespaces in a nested namespace declaration.
In this example:
namespace <X>::<Y>::<Z> {}
Only <Y> and <Z> will be classified as nested.

This flag isn't meant for assisting in building the AST, more for static analysis and refactorings.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D90568