CppEditor: Fully handle raw string literals in the syntax highlighter

As of a3af941adf, the built-in highlighter
can properly handle multi-line raw string literals, so we don't need to
abuse the semantic highlighter for this anymore.

Fixes: QTCREATORBUG-26693
Fixes: QTCREATORBUG-28284
Change-Id: If644767dfa8a97294e84a541eea44143e8d1bb88
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/testcases/highlightingtestcase.cpp

@@ -0,0 +1,7 @@
+auto func()
+{
+    return R"(foo
+    foobar
+        R"notaprefix!(
+    barfoobar)" R"(second)" /* comment */ R"(third)";
+}