[clangd] Disable background-index on lit-tests by default

[kadircet]

Summary

Since background-index can perform disk writes, we don't want to turn
it on tests that won't clear it.

[sammccall]

clang-tools-extra/clangd/tool/ClangdMain.cpp

361

maybe add "if enabled" here, this reads a little funny otherwise!

clang-tools-extra/clangd/tool/ClangdMain.cpp

llvm::cl::ParseCommandLineOptions(
       "For more information, see:"
       "\n\thttps://clang.llvm.org/extra/clangd.html"
       "\n\thttps://microsoft.github.io/language-server-protocol/");
   if (Test) {
     Sync = true;
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;
     PrettyPrint = true;
+    // Disable background index on lit tests by default to prevent disk writes.
+    if (!EnableBackgroundIndex.getNumOccurrences())
+      EnableBackgroundIndex = false;
     // Ensure background index makes progress.
     BackgroundQueue::preventThreadStarvationInTests();
   }
   if (Test || EnableTestScheme) {
     static URISchemeRegistry::Add<TestScheme> X(
         "test", "Test scheme for clangd lit tests.");
   }
 
   if (!Sync && WorkerThreadsCount == 0) {