Clang: Make file ids unique

Clang file ids are only unique for one query. Because we query in parallel
we have to manage our own unique ids.

Change-Id: I67d57d8b1766cab75ad252a14e57bbf9dc5fdb79
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/clangbackendipc/clangbackendipc-lib.pri

@@ -155,6 +155,7 @@ HEADERS += \
     $$PWD/pchmanagerclientproxy.h \
     $$PWD/projectpartpch.h \
     $$PWD/precompiledheadersupdatedmessage.h \
+    $$PWD/stringcache.h \
     $$PWD/removepchprojectpartsmessage.h \
     $$PWD/clangcodemodelclientmessages.h \
     $$PWD/clangcodemodelservermessages.h \