[clangd] Enable cross-namespace completions by default in clangd

[ioeric]

Summary

Code completion will suggest symbols from any scope (incl. inaccessible
scopes) when there's no qualifier explicitly specified. E.g.

As we are assigning relatively low scores for cross-namespace completion items, the overall code completion quality doesn't regress. The feature has been tried out by a few folks, and the feedback is generally positive, so I think it should be ready to be enabled by default.

[kadircet]

clangd/tool/ClangdMain.cpp

144

why not keep it hidden ?

[ioeric]

clangd/tool/ClangdMain.cpp

144

We hid the flag because the feature was experimental. Now that it's the default, I think it makes sense to expose the flag.

[kadircet]

clangd/tool/ClangdMain.cpp

144

I wouldn't expect anyone to turn it off, but not that important.

[ilya-biryukov]

LGTM. When we know everyone is happy with the new behaivour, we can remove the flag altogether.

clangd/tool/ClangdMain.cpp

144

+1 to making it visible. Now that actually enable it by default, it would be nice if the users could find it and turn it off in case they need it.

clangd/tool/ClangdMain.cpp

 
 static cl::opt<bool> AllScopesCompletion(
     "all-scopes-completion",
     cl::desc(
         "If set to true, code completion will include index symbols that are "
         "not defined in the scopes (e.g. "
         "namespaces) visible from the code completion point. Such completions "
         "can insert scope qualifiers."),
-    cl::init(false), cl::Hidden);
+    cl::init(true));
 
 static cl::opt<bool>
     ShowOrigins("debug-origin", cl::desc("Show origins of completion items"),
                 cl::init(clangd::CodeCompleteOptions().ShowOrigins),
                 cl::Hidden);
 
 static cl::opt<bool> HeaderInsertionDecorators(
     "header-insertion-decorators",