Add policy CMP0069 to enforce INTERPROCEDURAL_OPTIMIZATION

Previously the `INTERPROCEDURAL_OPTIMIZATION` target property was
honored only for the Intel compiler on Linux and otherwise ignored.  In
order to add support for more compilers incrementally without changing
behavior in the future, add a new policy whose NEW behavior enforces the
`INTERPROCEDURAL_OPTIMIZATION` property.  Add flags for supported
compilers and otherwise produce an error.

Source/cmGlobalGenerator.h

@@ -333,6 +333,8 @@ public:
 
   virtual bool UseFolderProperty() const;
 
+  virtual bool IsIPOSupported() const { return false; }
+
   /** Return whether the generator should use EFFECTIVE_PLATFORM_NAME. This is
       relevant for mixed macOS and iOS builds. */
   virtual bool UseEffectivePlatformName(cmMakefile*) const { return false; }