CppTools: CppPreprocessor: Read file only if necessary
CppTools: CppPreprocessor: Read file only if necessary

Change-Id: Ia60d3e1907f9238f873f016d844095084b76ad6b
Reviewed-by: Erik Verbruggen <5edfe905d082749f2ac3f4e6b42937f39f083771@digia.com>

colede/qtcreator,maui-packages/qt-creator,Distrotech/qtcreator,omniacreator/qtcreator,danimo/qt-creator,martyone/sailfish-qtcreator,farseerri/git_code,xianian/qt-creator,kuba1/qtcreator,kuba1/qtcreator,Distrotech/qtcreator,martyone/sailfish-qtcreator,farseerri/git_code,AltarBeastiful/qt-creator,xianian/qt-creator,farseerri/git_code,darksylinc/qt-creator,darksylinc/qt-creator,Distrotech/qtcreator,maui-packages/qt-creator,danimo/qt-creator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,martyone/sailfish-qtcreator,maui-packages/qt-creator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,maui-packages/qt-creator,AltarBeastiful/qt-creator,xianian/qt-creator,colede/qtcreator,kuba1/qtcreator,danimo/qt-creator,xianian/qt-creator,Distrotech/qtcreator,kuba1/qtcreator,maui-packages/qt-creator,martyone/sailfish-qtcreator,darksylinc/qt-creator,amyvmiwei/qt-creator,darksylinc/qt-creator,martyone/sailfish-qtcreator,danimo/qt-creator,farseerri/git_code,farseerri/git_code,colede/qtcreator,AltarBeastiful/qt-creator,danimo/qt-creator,amyvmiwei/qt-creator,omniacreator/qtcreator,omniacreator/qtcreator,darksylinc/qt-creator,xianian/qt-creator,danimo/qt-creator,AltarBeastiful/qt-creator,colede/qtcreator,kuba1/qtcreator,kuba1/qtcreator,kuba1/qtcreator,Distrotech/qtcreator,omniacreator/qtcreator,AltarBeastiful/qt-creator,amyvmiwei/qt-creator,farseerri/git_code,amyvmiwei/qt-creator,maui-packages/qt-creator,amyvmiwei/qt-creator,amyvmiwei/qt-creator,xianian/qt-creator,danimo/qt-creator,darksylinc/qt-creator,Distrotech/qtcreator,kuba1/qtcreator,xianian/qt-creator,colede/qtcreator,danimo/qt-creator,danimo/qt-creator,colede/qtcreator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,farseerri/git_code,xianian/qt-creator,darksylinc/qt-creator,omniacreator/qtcreator,Distrotech/qtcreator,xianian/qt-creator,colede/qtcreator,martyone/sailfish-qtcreator,AltarBeastiful/qt-creator,omniacreator/qtcreator,darksylinc/qt-creator,omniacreator/qtcreator,farseerri/git_code,kuba1/qtcreator,amyvmiwei/qt-creator,maui-packages/qt-creator