VS15: Add Visual Studio 15 generator

Call the generator "Visual Studio 15" without any year because the
preview version of VS 15 does not provide a year in the product name.

Copy cmGlobalVisualStudio14Generator to cmGlobalVisualStudio15Generator
and update version numbers accordingly.  Add the VS15 enumeration value.
Note that we do not need to add a MSVC15 variable or v150 toolset
because Visual Studio 15 comes with an updated version of the v140
toolset and remains ABI-compatible.

Teach tests VSExternalInclude, RunCMake.GeneratorPlatform, and
RunCMake.GeneratorToolset to treat VS 15 as they do VS 10-14.

Closes: #16143

Source/cmGlobalVisualStudio7Generator.cxx

@@ -163,6 +163,18 @@ std::string cmGlobalVisualStudio7Generator::FindDevEnvCommand()
     }
   }
 
+  // Search where VS15Preview places it.
+  vskey = "HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\VisualStudio\\SxS\\VS7;";
+  vskey += this->GetIDEVersion();
+  if (cmSystemTools::ReadRegistryValue(vskey.c_str(), vscmd,
+                                       cmSystemTools::KeyWOW64_32)) {
+    cmSystemTools::ConvertToUnixSlashes(vscmd);
+    vscmd += "/Common7/IDE/devenv.com";
+    if (cmSystemTools::FileExists(vscmd, true)) {
+      return vscmd;
+    }
+  }
+
   vscmd = "devenv.com";
   return vscmd;
 }