[clangd] Debounce rebuilds responsively to rebuild times.

[sammccall]

Summary

Old: 500ms always. New: rebuild time, up to 500ms.

Fixes https://github.com/clangd/clangd/issues/275

[merge_guards_bot]

Unit tests: pass. 62438 tests passed, 0 failed and 845 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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clang-tools-extra/clangd/ClangdServer.h

struct Options {
 
     /// The resource directory is used to find internal headers, overriding
     /// defaults and -resource-dir compiler flag).
     /// If None, ClangdServer calls CompilerInvocation::GetResourcePath() to
     /// obtain the standard resource directory.
     llvm::Optional<std::string> ResourceDir = llvm::None;
 
     /// Time to wait after a new file version before computing diagnostics.
-    DebouncePolicy UpdateDebounce =
-        DebouncePolicy::fixed(std::chrono::milliseconds(500));
+    DebouncePolicy UpdateDebounce = DebouncePolicy{
+        /*Min=*/std::chrono::milliseconds(50),
+        /*Max=*/std::chrono::milliseconds(500),
+        /*RebuildRatio=*/1,
+    };
 
     bool SuggestMissingIncludes = false;
 
     /// Clangd will execute compiler drivers matching one of these globs to
     /// fetch system include path.
     std::vector<std::string> QueryDriverGlobs;
 
     /// Enable semantic highlighting features.