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/qmakeparser.cpp

@@ -210,6 +210,12 @@ ProFile *QMakeParser::parsedProBlock(
     return pro;
 }
 
+void QMakeParser::discardFileFromCache(const QString &fileName)
+{
+    if (m_cache)
+        m_cache->discardFile(fileName);
+}
+
 bool QMakeParser::read(ProFile *pro)
 {
     QFile file(pro->fileName());