[clang-tools-extra] Improve loading speed of indexes used by clangd (PR #156185)

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<!--LLVM CODE FORMAT COMMENT: {clang-format}-->


:warning: C/C++ code formatter, clang-format found issues in your code. :warning:

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<summary>
You can test this locally with the following command:
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``````````bash
git-clang-format --diff origin/main HEAD --extensions h,cpp -- clang-tools-extra/clangd/index/FileIndex.cpp clang-tools-extra/clangd/index/Ref.cpp clang-tools-extra/clangd/index/Ref.h clang-tools-extra/clangd/index/Serialization.cpp clang-tools-extra/clangd/index/YAMLSerialization.cpp
``````````

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changing `origin/main` to the base branch/commit you want to compare against.
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<details>
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View the diff from clang-format here.
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``````````diff
diff --git a/clang-tools-extra/clangd/index/Ref.h b/clang-tools-extra/clangd/index/Ref.h
index 9a7e7aadb..06486cb59 100644
--- a/clang-tools-extra/clangd/index/Ref.h
+++ b/clang-tools-extra/clangd/index/Ref.h
@@ -166,6 +166,7 @@ public:
     void insert(const SymbolID &ID, const Ref &S);
     /// Consumes the builder to finalize the slab.
     RefSlab build() &&;
+
   private:
     // A ref we're storing with its symbol to consume with build().
     // All strings are interned, so DenseMapInfo can use pointer comparisons.

``````````

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https://github.com/llvm/llvm-project/pull/156185