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

[Mythreya Kuricheti via cfe-commits]

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@@ -460,6 +462,17 @@ DirectoryBasedGlobalCompilationDatabase::lookupCDB(
   return Result;
 }
 
+void DirectoryBasedGlobalCompilationDatabase::Options::applyWorkingDirectory(
+    const std::optional<std::string> &&WorkingDirectory) {
+  if (WorkingDirectory)
+    this->WorkingDirectory = *WorkingDirectory;
+  else {
+    SmallString<256> CWD;
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MythreyaK wrote:

I was wondering if an error instead of a silent fallback helps with immediate user feedback, since this is an opt-in feature. Strong workspace's precondition is that a the directory should be set anyway. 

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