Clang: implement followSymbol in TranslationUnit

Follow symbol in current TU or dependent files
Current algorithm tries to do the same as built-in
follow symbol but better.
Currently clang-based follow symbol has some limitations:
- following function usage may return the declaration
instead of definition because we don't have header
dependencies in backend
- overrides are not searched because of the same reason
and the amount of dependent files (parsing 250 files
takes a while)
- some includes are not handled correctly, in that case
we return failure and ask built-in code model to follow
(example: <QtGui> or other qt includes)

Change-Id: If35028ee0b5e818fdba29363c9520c5cca996348
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/tools/clangbackend/ipcsource/clangtranslationunit.h

@@ -27,12 +27,8 @@
 
 #include <clangbackendipc/codecompletion.h>
 
-#include <utf8string.h>
-
 #include <clang-c/Index.h>
 
-class Utf8String;
-
 namespace ClangBackEnd {
 
 class Cursor;
@@ -41,6 +37,7 @@ class DiagnosticSet;
 class HighlightingMarkContainer;
 class HighlightingMarks;
 class ReferencesResult;
+class FollowSymbolResult;
 class SkippedSourceRanges;
 class SourceLocation;
 class SourceRange;
@@ -48,6 +45,7 @@ class SourceRangeContainer;
 class TranslationUnitUpdateInput;
 class TranslationUnitUpdateResult;
 class UnsavedFiles;
+class CommandLineArguments;
 
 class TranslationUnit
 {
@@ -102,6 +100,10 @@ public:
     HighlightingMarks highlightingMarksInRange(const SourceRange &range) const;
 
     SkippedSourceRanges skippedSourceRanges() const;
+    FollowSymbolResult followSymbol(uint line,
+                                    uint column,
+                                    const QVector<Utf8String> &dependentFiles,
+                                    const CommandLineArguments &currentArgs) const;
 
 private:
     const Utf8String m_id;