Align the full version schema across all artifacts

Summary:
The version number used for packaging symbols and xswiftbus was not the
full one including the timestamp.
This change adds the last commit timestamp (in contrast to the build
timestamp used before) as regular part of the version number. This helps
us to also keep the version number consistent with the content across all
installers. The version number will change only, if the last commit was
modified and not with each different build.
In the end, all jenkins jobs building from the same commit should produce
artifacts with the same version everywhere.

ref T204

Reviewers: #swift_pilot_client, kbasan

Reviewed By: #swift_pilot_client, kbasan

Subscribers: jenkins

Maniphest Tasks: T204

Differential Revision: https://dev.swift-project.org/D68

src/blackconfig/buildconfig.h

@@ -119,6 +119,9 @@ namespace BlackConfig
         //! Returns SHA-1 of git HEAD at build time
         static const QString &gitHeadSha1();
 
+        //! Timestamp of the last commit (NOT the authored timestamp)
+        static const QDateTime &lastCommitTimestamp(); // defined in buildconfig_gen.cpp.in
+
         //! Build timestamp
         static const QDateTime &buildTimestamp();
 
@@ -131,8 +134,8 @@ namespace BlackConfig
         //! Version as QVersionNumber
         static const QString &getVersionString();
 
-        //! Turns build timestamp into a version number
-        static int buildTimestampAsVersionSegment(const QDateTime &buildTimestamp);
+        //! Turns last commit timestamp into a version number
+        static int lastCommitTimestampAsVersionSegment(const QDateTime &lastCommitTimestamp);
 
         //! Build ABI parts as in http://doc.qt.io/qt-5/qsysinfo.html#buildAbi
         static const QStringList &getBuildAbiParts();