ProjectExplorer: Upstream CMake's handling of outdated builds

This is mainly there to make the local runconfiguration implementations
more uniform. Both qmake and qbs will drop runconfigs for builds that
are not available anymore, so that's a no-op for them.

Change-Id: I8aa32d779f67cce7a4d4733cfbe0c9a136bfd3f7
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/target.cpp

@@ -365,6 +365,13 @@ BuildTargetInfo Target::buildTarget(const QString &buildKey) const
     });
 }
 
+bool Target::hasBuildTarget(const QString &buildKey) const
+{
+    return Utils::anyOf(d->m_appTargets.list, [buildKey](const BuildTargetInfo &bti) {
+        return bti.buildKey == buildKey;
+    });
+}
+
 QList<ProjectConfiguration *> Target::projectConfigurations() const
 {
     QList<ProjectConfiguration *> result;