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/cmExportInstallFileGenerator.cxx

@@ -398,7 +398,7 @@ cmExportInstallFileGenerator
   cmTarget* target = itgen->GetTarget();
 
   // Construct the installed location of the target.
-  std::string dest = itgen->GetDestination();
+  std::string dest = itgen->GetDestination(config);
   std::string value;
   if(!cmSystemTools::FileIsFullPath(dest.c_str()))
     {