[clangd] downgrade missing-includes diagnostic to Information level

[sammccall]

Summary

In practice, a Warning on every occurrence is very unpopular, even on a codebase
with clear rules about direct inclusion & moderately good compliance.

This change has various practical effects (in vscode for concreteness):

• makes the diagnostic decoration less striking (blue vs yellow)
• makes these diagnostics visually distinct from others when reading
• causes these diagnostics to sort last in the "problems" view
• allows these diagnostics to be easily filtered from the "problems" view

[sammccall]

This change is just one option to be evaluated for making these diagnostics more user-friendly.

Mostly sending the patch now to attach some screenshots...
Effect in VSCode (blue underline, visible in problems view, filterable)

"Hint" severity in VSCode (ellipsis, not visible in problems view)

(This seems worse: the three-dots rendering does suggest "action here" rather than "issue here", and not being *able* to show these in the problems view seems unfortunate too)

[kadircet]

thanks, lgtm!

clang-tools-extra/clangd/IncludeCleaner.cpp

for (const auto &SymbolWithMissingInclude : MissingIncludes) {
     Diag &D = Result.emplace_back();
     D.Message =
         llvm::formatv("No header providing \"{0}\" is directly included",
                       getSymbolName(SymbolWithMissingInclude.Symbol));
     D.Name = "missing-includes";
     D.Source = Diag::DiagSource::Clangd;
     D.File = AST.tuPath();
     D.InsideMainFile = true;
-    D.Severity = DiagnosticsEngine::Warning;
+    // We avoid the "warning" severity here in favor of LSP's "information".
+    //
+    // Users treat most warnings on code being edited as high-priority.
+    // They don't think of include cleanups the same way: they want to edit
+    // lines with existing violations without fixing them.
+    // Diagnostics at the same level tend to be visually indistinguishable,
+    // and a few missing includes can cause many diagnostics.
+    // Marking these as "information" leaves them visible, but less intrusive.
+    //
+    // (These concerns don't apply to unused #include warnings: these are fewer,
+    // they appear on infrequently-edited lines with few other warnings, and
+    // the 'Unneccesary' tag often result in a different rendering)
+    //
+    // Usually clang's "note" severity usually has special semantics, being
+    // translated into LSP RelatedInformation of a parent diagnostic.
+    // But not here: these aren't processed by clangd's DiagnosticConsumer.
+    D.Severity = DiagnosticsEngine::Note;
     D.Range = clangd::Range{
         offsetToPosition(Code,
                          SymbolWithMissingInclude.SymRefRange.beginOffset()),
         offsetToPosition(Code,
                          SymbolWithMissingInclude.SymRefRange.endOffset())};
     auto &F = D.Fixes.emplace_back();
     F.Message = "#include " + Spelling;
     TextEdit Edit = replacementToEdit(Code, *Replacement);

clang-tools-extra/clangd/test/include-cleaner-batch-fix.test

 # CHECK-NEXT:             "character": 4,
 # CHECK-NEXT:             "line": 2
 # CHECK-NEXT:           },
 # CHECK-NEXT:           "start": {
 # CHECK-NEXT:             "character": 1,
 # CHECK-NEXT:             "line": 2
 # CHECK-NEXT:           }
 # CHECK-NEXT:         },
-# CHECK-NEXT:         "severity": 2,
+# CHECK-NEXT:         "severity": 3,
 # CHECK-NEXT:         "source": "clangd"
 # CHECK-NEXT:       },
 # CHECK-NEXT:       {
 # CHECK-NEXT:         "code": "missing-includes",
 # CHECK-NEXT:         "message": "No header providing \"Bar\" is directly included (fixes available)",
 # CHECK-NEXT:         "range": {
 # CHECK-NEXT:           "end": {
 # CHECK-NEXT:             "character": 14,
 # CHECK-NEXT:             "line": 2
 # CHECK-NEXT:           },
 # CHECK-NEXT:           "start": {
 # CHECK-NEXT:             "character": 11,
 # CHECK-NEXT:             "line": 2
 # CHECK-NEXT:           }
 # CHECK-NEXT:         },
-# CHECK-NEXT:         "severity": 2,
+# CHECK-NEXT:         "severity": 3,
 # CHECK-NEXT:         "source": "clangd"
 # CHECK-NEXT:       },
 # CHECK-NEXT:       {
 # CHECK-NEXT:         "code": "unused-includes",
 # CHECK-NEXT:         "codeDescription": {
 # CHECK-NEXT:           "href": "https://clangd.llvm.org/guides/include-cleaner"
 # CHECK-NEXT:         },
 # CHECK-NEXT:         "message": "Included header all1.h is not used directly (fixes available)",

 # CHECK-NEXT:           1
 # CHECK-NEXT:         ]
 # CHECK-NEXT:       }
 # CHECK-NEXT:     ],
 # CHECK-NEXT:     "uri": "file://{{.*}}/simple.cpp",
 # CHECK-NEXT:     "version": 0
 # CHECK-NEXT:   }
 ---
-{"jsonrpc":"2.0","id":2,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///simple.cpp"},"range":{"start":{"line":2,"character":1},"end":{"line":2,"character":4}},"context":{"diagnostics":[{"range":{"start": {"line": 2, "character": 1}, "end": {"line": 2, "character": 4}},"severity":2,"message":"No header providing \"Foo\" is directly included (fixes available)", "code": "missing-includes", "source": "clangd"}]}}}
+{"jsonrpc":"2.0","id":2,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///simple.cpp"},"range":{"start":{"line":2,"character":1},"end":{"line":2,"character":4}},"context":{"diagnostics":[{"range":{"start": {"line": 2, "character": 1}, "end": {"line": 2, "character": 4}},"severity":3,"message":"No header providing \"Foo\" is directly included (fixes available)", "code": "missing-includes", "source": "clangd"}]}}}
 #      CHECK:  "id": 2,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:  "result": [
 # CHECK-NEXT:    {
 # CHECK-NEXT:      "arguments": [
 # CHECK-NEXT:        {
 # CHECK-NEXT:          "documentChanges": [
 # CHECK-NEXT:            {