ClangCodeModel: Suppress "Unknown argument" error

Unknown compiler flags are an unsuppressable error in clang.
But we do not want to maintain a blacklist of unsupported GCC and MSVC
options or a whitelist of supported clang options, as both would
constantly go out of date.
As clangd seems to work fine despite the error message, we simply filter
out this type of diagnostic.

Fixes: QTCREATORBUG-27113
Change-Id: Ib32601831eded60daf80eb0ca5cf01bbd71493fa
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdclient.cpp

@@ -1321,7 +1321,12 @@ ClangdClient::ClangdClient(Project *project, const Utils::FilePath &jsonDbDir)
         return new ClangdTextMark(filePath, diag, isProjectFile, this);
     };
     const auto hideDiagsHandler = []{ ClangDiagnosticManager::clearTaskHubIssues(); };
-    setDiagnosticsHandlers(textMarkCreator, hideDiagsHandler);
+    static const auto diagsFilter = [](const Diagnostic &diag) {
+        const Diagnostic::Code code = diag.code().value_or(Diagnostic::Code());
+        const QString * const codeString = Utils::get_if<QString>(&code);
+        return !codeString || *codeString != "drv_unknown_argument";
+    };
+    setDiagnosticsHandlers(textMarkCreator, hideDiagsHandler, diagsFilter);
     setSymbolStringifier(displayNameFromDocumentSymbol);
     setSemanticTokensHandler([this](TextDocument *doc, const QList<ExpandedSemanticToken> &tokens,
                                     int version, bool force) {

src/plugins/clangcodemodel/test/data/completion/completion.pro

@@ -27,5 +27,7 @@ SOURCES = \
     privateFuncDefCompletion.cpp \
     signalCompletion.cpp
 
+QMAKE_CXXFLAGS += -ffoo
+
 HEADERS = mainwindow.h
 FORMS = mainwindow.ui

src/plugins/clangcodemodel/test/data/find-usages/find-usages.pro

@@ -2,3 +2,4 @@ TEMPLATE = app
 QT = core
 HEADERS = defs.h
 SOURCES = main.cpp
+QMAKE_CXXFLAGS += -ffoo

src/plugins/clangcodemodel/test/data/follow-symbol/follow-symbol.pro

@@ -2,3 +2,4 @@ TEMPLATE = app
 CONFIG -= qt
 HEADERS = cursor.h header.h
 SOURCES = cursor.cpp main.cpp
+QMAKE_CXXFLAGS += -ffoo

src/plugins/clangcodemodel/test/data/highlighting/highlighting.pro

@@ -1,3 +1,4 @@
 TEMPLATE = app
 CONFIG -= qt
 SOURCES = highlighting.cpp
+QMAKE_CXXFLAGS += -ffoo

src/plugins/clangcodemodel/test/data/local-references/local-references.pro

@@ -1,5 +1,4 @@
 TEMPLATE = app
-
 CONFIG -= qt
-
 SOURCES = references.cpp
+QMAKE_CXXFLAGS += -ffoo