CppTools: Extract base class CppCompletionAssistProcessor

As as side effect, this also brings some more pragma completions for the
builtin-in completion engine, e.g. "pragma once" or "pragma omp atomic".

Change-Id: If3ef22076c331c653b78a87cfff836c1da38c8fb
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/cpptools/cpptools.pro

@@ -22,6 +22,7 @@ HEADERS += \
     cppcodestylesettings.h \
     cppcodestylesettingspage.h \
     cppcompletionassist.h \
+    cppcompletionassistprocessor.h \
     cppcompletionassistprovider.h \
     cppcurrentdocumentfilter.h \
     cppdoxygen.h \
@@ -84,6 +85,7 @@ SOURCES += \
     cppcodestylesettings.cpp \
     cppcodestylesettingspage.cpp \
     cppcompletionassist.cpp \
+    cppcompletionassistprocessor.cpp \
     cppcompletionassistprovider.cpp \
     cppcurrentdocumentfilter.cpp \
     cppdoxygen.cpp \