Fix qmake's pro file cache to interact correctly with write_file

When writing a file with write_file() we have to inform the pro file parser
cache to discard the file if it's existent in the cache, to ensure that
calling include() after write_file() always works.

Change-Id: I7d09269a57de55ca30b0e11dd40770de9f919f64
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/f45d6414ad6ef1d75f049a61411cd954bcae29c3)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/shared/proparser/qmakebuiltins.cpp

@@ -308,6 +308,7 @@ QMakeEvaluator::writeFile(const QString &ctx, const QString &fn, QIODevice::Open
                   .arg(ctx, QDir::toNativeSeparators(qfi.filePath()), errStr));
         return ReturnFalse;
     }
+    m_parser->discardFileFromCache(qfi.filePath());
     return ReturnTrue;
 }