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

[njames93]

Summary

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

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

[Mordante]

Just a few minor nits. I'll leave the approving to the other reviewers.

clang/lib/AST/TextNodeDumper.cpp

1828

Can you also modify JSONNodeDumper::VisitNamespaceDecl ?

clang/lib/Sema/SemaDeclCXX.cpp

11169

Please change false to /*Nested=*/false.

[shafik]

clang/lib/AST/ASTContext.cpp

7866

As noted elsewhere instead of just false please use /*Nested*/ false

Also a little below.

[njames93]

Added JsonNodeDumper and argument comments.

[Mordante]

In D90568#2370079, @njames93 wrote:

Added JsonNodeDumper and argument comments.

Thanks, I've no further comments.

[njames93]

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