Wizards: Support using #pragma once instead of include guards

Allow users to choose #pragma once instead of #ifndef include guards in
generated header files.

Fixes: QTCREATORBUG-12166
Change-Id: I3ba41c7570beb9c5958e174b5581fcc25855050f
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Filip Bucek <fbucek@atlas.cz>

src/plugins/cpptools/cpptoolsplugin.cpp

@@ -140,6 +140,11 @@ QString CppToolsPlugin::licenseTemplate()
     return m_instance->m_fileSettings->licenseTemplate();
 }
 
+bool CppToolsPlugin::usePragmaOnce()
+{
+    return m_instance->m_fileSettings->headerPragmaOnce;
+}
+
 const QStringList &CppToolsPlugin::headerSearchPaths()
 {
     return m_instance->m_fileSettings->headerSearchPaths;
@@ -204,6 +209,11 @@ bool CppToolsPlugin::initialize(const QStringList &arguments, QString *error)
                                     tr("The configured path to the license template"),
                                     []() { return CppToolsPlugin::licenseTemplatePath().toString(); });
 
+    expander->registerVariable(
+                "Cpp:PragmaOnce",
+                tr("Insert #pragma once instead of #ifndef include guards into header file"),
+                [] { return usePragmaOnce() ? QString("true") : QString(); });
+
     return true;
 }