[clang] [include-cleaner] Support multi-header symbols in mapping generator (PR #147720)

[Ildikó Cseri via cfe-commits]

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@@ -232,4 +247,4 @@ def GetSymbols(parse_pages):
     finally:
         pool.terminate()
         pool.join()
-    return sorted(symbols)
+    return sorted(set(symbols))
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cseriildi wrote:

Some symbols are on multiple pages, like `wstring_convert` on `symbol_index.html` and also on the `zombie_names.html`, but they point to the same `locale/wstring_convert.html`, and also many more for c on `index.html` and also on `macro.html`.

https://github.com/llvm/llvm-project/pull/147720