Add support for Visual Studio 15.0 (MSVC2017)

Extend ABI by flavor and add look-up.

As with each major update, rewrite the the MSVC toolchain
autodetection.
Previously, the detection looped over the entries in "VC" registry
section and checked in the "VS" section whether a valid installation
belonged to the entry. MSVC2017 no longer has an entry in the "VC"
section.
Introduce a struct describing the installation including version
number and rewrite the loop to run over the "VS" section and retrieve
the VC path from the "VC" section only for versions <= 14.0.
Also, the various .bat files for the target architecture are
now in a single folder instead of spread around various bin/XXX
subfolders. Introduce an array of structs for the platform that
has the prefix and .bat name and uses the prefix only for
versions <= 14.0.

Task-number: QTBUG-57086
Change-Id: I947edd885c0a379f196440041c07d60cd4244d9c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/abi.cpp

@@ -695,6 +695,8 @@ QString Abi::toString(const OSFlavor &of)
         return QLatin1String("msvc2013");
     case Abi::WindowsMsvc2015Flavor:
         return QLatin1String("msvc2015");
+    case Abi::WindowsMsvc2017Flavor:
+        return QLatin1String("msvc2017");
     case Abi::WindowsMSysFlavor:
         return QLatin1String("msys");
     case Abi::WindowsCEFlavor: