[clangd] Turn background-index on by default

[sammccall]

Great! please land once D64018 is done.

[hokein]

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

191

should we remove the experimental term here?

[kadircet]

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

191

i would rather do that if we are happy with the feedbacks(or no feedbacks) within the following weeks

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

llvm::cl::desc(
         "WARNING: This option is experimental only, and will be removed "
         "eventually. Don't rely on it"),
     llvm::cl::init(""), llvm::cl::Hidden);
 
 static llvm::cl::opt<bool> EnableBackgroundIndex(
     "background-index",
     llvm::cl::desc(
         "Index project code in the background and persist index on disk. "
         "Experimental"),
-    llvm::cl::init(false), llvm::cl::Hidden);
+    llvm::cl::init(true));
 
 static llvm::cl::opt<int> BackgroundIndexRebuildPeriod(
     "background-index-rebuild-period",
     llvm::cl::desc(
         "If set to non-zero, the background index rebuilds the symbol index "
         "periodically every X milliseconds; otherwise, the "
         "symbol index will be updated for each indexed file"),
     llvm::cl::init(5000), llvm::cl::Hidden);