[clang-tools-extra] [clangd] Make inline friend functions appear in document symbols (PR #150629)

[kadir çetinkaya via cfe-commits]

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@@ -391,6 +392,16 @@ class DocumentOutline {
         D = TD;
     }
 
+    // Friend declarations should be traversed if and only if
+    // they are also defined here.
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kadircet wrote:

we aren't really visiting friend decl, we're visiting the underlying function definition.

can you also leave a comment mentioning why we special case this? (e.g. `FriendDecls don't act as `DeclContext`s but they might wrap a function definition that won't be visible through other means in the AST. Hence unwrap it here instead.`)

https://github.com/llvm/llvm-project/pull/150629