[clang-tools-extra] Add --project-root to clangd (PR #155905)

[Nathan Ridge via cfe-commits]

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@@ -69,14 +71,17 @@ class GlobalCompilationDatabase {
   }
 
 protected:
+  std::optional<std::string> WorkingDirectory;
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HighCommander4 wrote:

Since this only applies to fallback commands, let's call it `FallbackWorkingDirectory`. Likewise for the field in `DirectoryBasedGlobalCompilationDatabase::Options`, the method that sets it, etc.

https://github.com/llvm/llvm-project/pull/155905