install: Allow generator expressions in TARGETS DESTINATION (#14317)

This will allow per-config destinations for targets in EXPORT sets.
Using multiple install(TARGETS) with separate CONFIGURATIONS is
rejected as a target appearing more than once in an export set.
Now instead one can write

 install(TARGETS foo EXPORT exp DESTINATION lib/$<CONFIG>)

to get a single logical membership of the target in the export set
while still having a per-config destination.

Source/cmInstallTargetGenerator.h

@@ -60,8 +60,7 @@ public:
   cmTarget* GetTarget() const { return this->Target; }
   bool IsImportLibrary() const { return this->ImportLibrary; }
 
-  std::string const& GetDestination() const
-    { return this->Destination; }
+  std::string GetDestination(std::string const& config) const;
 
 protected:
   virtual void GenerateScript(std::ostream& os);