[clang-tools-extra] [clang-doc] Serialize an `Anonymous` JSON field for enums and records (PR #174265)

[Erick Velez via cfe-commits]

evelez7 wrote:

Before: https://erickvelez.com/clang-doc-mustache-output/pr174265-before/GlobalNamespace/index.html
After: https://erickvelez.com/clang-doc-mustache-output/pr174265/GlobalNamespace/index.html

Note the extraneous `Global Namespace` in the sidebar in the "before" link. Since the anonymous enum had no name, it took the name of the outer context which is the global namespace.

https://github.com/llvm/llvm-project/pull/174265