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/clangbackendclangipc-source.pri

@@ -16,6 +16,8 @@ HEADERS += \
     $$PWD/clangexceptions.h \
     $$PWD/clangfilepath.h \
     $$PWD/clangfilesystemwatcher.h \
+    $$PWD/clangfollowsymboljob.h \
+    $$PWD/clangfollowsymbol.h \
     $$PWD/clangiasyncjob.h \
     $$PWD/clangjobcontext.h \
     $$PWD/clangjobqueue.h \
@@ -55,8 +57,7 @@ HEADERS += \
     $$PWD/sourcerange.h \
     $$PWD/unsavedfile.h \
     $$PWD/unsavedfiles.h \
-    $$PWD/utf8positionfromlinecolumn.h \
-    $$PWD/clangfollowsymboljob.h
+    $$PWD/utf8positionfromlinecolumn.h
 
 SOURCES += \
     $$PWD/clangcodecompleteresults.cpp \
@@ -71,6 +72,8 @@ SOURCES += \
     $$PWD/clangexceptions.cpp \
     $$PWD/clangfilepath.cpp \
     $$PWD/clangfilesystemwatcher.cpp \
+    $$PWD/clangfollowsymboljob.cpp \
+    $$PWD/clangfollowsymbol.cpp \
     $$PWD/clangiasyncjob.cpp \
     $$PWD/clangjobcontext.cpp \
     $$PWD/clangjobqueue.cpp \
@@ -107,5 +110,4 @@ SOURCES += \
     $$PWD/sourcerange.cpp \
     $$PWD/unsavedfile.cpp \
     $$PWD/unsavedfiles.cpp \
-    $$PWD/utf8positionfromlinecolumn.cpp \
-    $$PWD/clangfollowsymboljob.cpp
+    $$PWD/utf8positionfromlinecolumn.cpp