VS: Select and save a VS 2017 instance persistently

Visual Studio 2017 supports multiple instances installed on a single
machine.  We use the Visual Studio Installer tool to enumerate instances
and select one.  Once we select an instance for a given build tree, save
the result in `CMAKE_GENERATOR_INSTANCE` so we can re-configure the tree
with the same instance on future re-runs of CMake.

Fixes: #17268

Source/cmGlobalVisualStudio15Generator.h

@@ -27,6 +27,11 @@ public:
   virtual void WriteSLNHeader(std::ostream& fout);
 
   virtual const char* GetToolsVersion() { return "15.0"; }
+
+  bool SetGeneratorInstance(std::string const& i, cmMakefile* mf) override;
+
+  bool GetVSInstance(std::string& dir) const;
+
 protected:
   bool InitializeWindows(cmMakefile* mf) override;
   virtual bool SelectWindowsStoreToolset(std::string& toolset) const;