ClangCodeModel: Ignore built-in types

... in the semantic highlighter. The syntax highlighter already handles
these.

Change-Id: Ib1e80ec194021dbd2b4eca7a300db869ccddb7b0
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/testcases/highlightingtestcase.cpp

@@ -21,3 +21,7 @@ void numberLiterals()
 }
 
 template<int n = 5> class C;
+
+struct ConversionFunction {
+    operator int();
+};