Add new target-property IMPORTED_GLOBAL.

The purpose of this new `IMPORTED_GLOBAL` target-property is to prolong
the lifetime and scope of `IMPORTED` targets in such a way as if they
had been created with the keyword `GLOBAL` in the first place.

* It can only be set to `TRUE`. That means, a local `IMPORTED` target
  can be promoted to global scope but a global `IMPORTED` target cannot
  be degraded to local scope!
* Setting it to `TRUE` only succeeds if done from within the same
  directory in which the `IMPORTED` target was created in the first
  place.

Fixes #17256.

Source/cmTargetPropertyComputer.cxx

@@ -57,6 +57,7 @@ bool cmTargetPropertyComputer::WhiteListedInterfaceProperty(
     builtIns.insert("COMPATIBLE_INTERFACE_STRING");
     builtIns.insert("EXPORT_NAME");
     builtIns.insert("IMPORTED");
+    builtIns.insert("IMPORTED_GLOBAL");
     builtIns.insert("NAME");
     builtIns.insert("TYPE");
   }