VS: Update VS 15 generator for Preview 5

The `PlatformToolset` is now `v141` instead of `v140`.

Closes: #16347

Help/release/3.7.rst

@@ -32,7 +32,7 @@ Generators
   with :ref:`Makefile Generators`.
 
 * The :generator:`Visual Studio 15` generator was added.  This is
-  experimental and based on Preview 4 because this version of VS
+  experimental and based on Preview 5 because this version of VS
   has not been released.
 
 * :ref:`Visual Studio Generators` for VS 2010 and above learned to

Source/cmGlobalVisualStudio15Generator.cxx

@@ -79,7 +79,7 @@ cmGlobalVisualStudio15Generator::cmGlobalVisualStudio15Generator(
     "HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\VCExpress\\15.0\\Setup\\VC;"
     "ProductDir",
     vc15Express, cmSystemTools::KeyWOW64_32);
-  this->DefaultPlatformToolset = "v140";
+  this->DefaultPlatformToolset = "v141";
   this->Version = VS15;
 }