Clang: implement requestFollowSymbol plug-in side

Invoke follow symbol in clang backend
if env variable QTC_CLANG_FOLLOW_SYMBOL is 1.
Does not include backend implementation.

Change-Id: Ia20a677830ebdd7f24800af5c5d6e8b1bf579205
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/tools/clangbackend/ipcsource/clangjobrequest.h

@@ -103,6 +103,7 @@ public:
     quint32 line = 0;
     quint32 column = 0;
     quint64 ticketNumber = 0;
+    QVector<Utf8String> dependentFiles;
     bool resolveTarget = true;
 };