[clangd] Enable cross-file-rename by default.

[hokein]

Summary

The cross-file rename feature is stable enough to enable it (has been
rolled out internally for a few weeks).

[sammccall]

I suspect we're going to get some complaints from people whose index is perpetually behind and missing files, but this is the tradeoff of the design, we have to accept that or refine (e.g. with a textual index).

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

list<std::string> TweakList{
     desc("Specify a list of Tweaks to enable (only for clangd developers)."),
     Hidden,
     CommaSeparated,
 };
 
 opt<bool> CrossFileRename{
     "cross-file-rename",
     cat(Features),
-    desc("Enable cross-file rename feature. Note that this feature is "
-         "experimental and may lead to broken code or incomplete rename "
-         "results"),
-    init(false),
-    Hidden,
+    desc("Enable cross-file rename feature."),
+    init(true),
 };
 
 opt<bool> RecoveryAST{
     "recovery-ast",
     cat(Features),
     desc("Preserve expressions in AST for broken code (C++ only). Note that "
          "this feature is experimental and may lead to crashes"),
     init(false),