Genex: Evaluate byproduct generator expressions in cmCustomCommandGenerator

Evaluate and expand generator expressions in byproducts of custom commands.

Note that it is still not possible to use generator expressions in byproducts
of the commands `add_custom_command` and `add_custom_target`.  These commands
still reject the input.  This is a preparation step for OUTPUT generator
expression support.

Issue: #12877

Source/cmCustomCommandGenerator.cxx

@@ -72,6 +72,8 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(cmCustomCommand const& cc,
     this->CommandLines.push_back(std::move(argv));
   }
 
+  AppendPaths(cc.GetByproducts(), *this->GE, this->LG, this->Config,
+              this->Byproducts);
   AppendPaths(cc.GetDepends(), *this->GE, this->LG, this->Config,
               this->Depends);
 
@@ -248,7 +250,7 @@ std::vector<std::string> const& cmCustomCommandGenerator::GetOutputs() const
 
 std::vector<std::string> const& cmCustomCommandGenerator::GetByproducts() const
 {
-  return this->CC.GetByproducts();
+  return this->Byproducts;
 }
 
 std::vector<std::string> const& cmCustomCommandGenerator::GetDepends() const

Source/cmCustomCommandGenerator.h

@@ -23,6 +23,7 @@ class cmCustomCommandGenerator
   cmGeneratorExpression* GE;
   cmCustomCommandLines CommandLines;
   std::vector<std::vector<std::string>> EmulatorsWithArguments;
+  std::vector<std::string> Byproducts;
   std::vector<std::string> Depends;
   std::string WorkingDirectory;
 

Source/cmMakefileTargetGenerator.cxx

@@ -221,7 +221,8 @@ void cmMakefileTargetGenerator::WriteTargetBuildRules()
              this->GeneratorTarget->GetPostBuildCommands());
 
     for (const auto& be : buildEventCommands) {
-      const std::vector<std::string>& byproducts = be.GetByproducts();
+      cmCustomCommandGenerator beg(be, this->ConfigName, this->LocalGenerator);
+      const std::vector<std::string>& byproducts = beg.GetByproducts();
       for (std::string const& byproduct : byproducts) {
         this->CleanFiles.insert(
           this->LocalGenerator->MaybeConvertToRelativePath(currentBinDir,