Clang: Initialize HighlightingTypes to Invalid

It is too easy to forget to initialize the type, so it can have a wrong
value. Actually we should use exceptions elsewhere because this can be
simply hiding bugs.

Change-Id: I71c0b467c12409b489c173e3edc53cdaa406bf09
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/libs/clangsupport/clangsupport_global.h

@@ -154,7 +154,7 @@ struct MessageTrait<Message> \
 using MixinHighlightingTypes = Utils::SizedArray<HighlightingType, 6>;
 
 struct HighlightingTypes {
-    HighlightingType mainHighlightingType;
+    HighlightingType mainHighlightingType = HighlightingType::Invalid;
     MixinHighlightingTypes mixinHighlightingTypes;
 };