Clang: Use completion through backend process

This makes us independent of libclang crashes for completion.
Re-parsing for highlighting still happens in the Qt Creator process.

Run in verbose mode:
    qtc.clangcodemodel.ipc=true

Run tests:
    -test "ClangCodeModel"

Task-number: QTCREATORBUG-14108
Task-number: QTCREATORBUG-12819
Change-Id: Id3e95bd2afdb6508bbd1d35fddc69534a909b905
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/cpptools/editordocumenthandle.h

@@ -51,7 +51,9 @@ public:
     virtual unsigned revision() const = 0;
 
     // For updating if new project info is set
-    virtual BaseEditorDocumentProcessor *processor() = 0;
+    virtual BaseEditorDocumentProcessor *processor() const = 0;
+
+    virtual void resetProcessor() = 0;
 
 private:
     bool m_needsRefresh;