[Reg 4.0->4.1] Semantic highlighting (clang) freezes UI in larger files

commit f8e64a87bbe8f4c1cfa50af4652f76a145d2a84d
"Clang: Add Declaration highlighting"

results in SemanticHighlighter::onHighlighterResultAvailable taking a lot more time (when editing editormanager.cpp from Qt Creator, time spent there increases from 0-1ms to 3-4ms for each call, increasing the total amount from <30ms to almost 200ms), resulting in perceptible, annoying little freezes of the UI a short time after each edit.