CppEditor: Allow to retrieve clangd's internal include path

The code model will need to use this one instead of the one hardcoded at
build time.

Change-Id: Ibd89d32f99b9250c909473f9a4a66f9a8a5f4f07
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppcodemodelsettings.cpp

@@ -405,6 +405,19 @@ QVersionNumber ClangdSettings::clangdVersion(const FilePath &clangdFilePath)
     return it->second;
 }
 
+FilePath ClangdSettings::clangdIncludePath() const
+{
+    QTC_ASSERT(useClangd(), return {});
+    FilePath clangdPath = clangdFilePath();
+    QTC_ASSERT(!clangdPath.isEmpty() && clangdPath.exists(), return {});
+    const QVersionNumber version = clangdVersion();
+    QTC_ASSERT(!version.isNull(), return {});
+    const FilePath includePath = clangdPath.absolutePath().parentDir().pathAppended("lib/clang")
+            .pathAppended(version.toString()).pathAppended("include");
+    QTC_ASSERT(includePath.exists(), return {});
+    return includePath;
+}
+
 void ClangdSettings::loadSettings()
 {
     Utils::fromSettings(clangdSettingsKey(), {}, Core::ICore::settings(), &m_data);

src/plugins/cppeditor/cppcodemodelsettings.h

@@ -155,6 +155,7 @@ public:
 
     static QVersionNumber clangdVersion(const Utils::FilePath &clangdFilePath);
     QVersionNumber clangdVersion() const { return clangdVersion(clangdFilePath()); }
+    Utils::FilePath clangdIncludePath() const;
 
 #ifdef WITH_TESTS
     static void setUseClangd(bool use);