cmSystemTools: Add ExpandedListArgument and ExpandedLists methods

Changes
-------

In `cmSystemTools` this
- renames the method `ExpandList` to `ExpandLists` and makes it iterator based

and adds the methods
- `std::vector<std::string> ExpandedLists(InputIt first, InputIt last)`
- `std::vector<std::string> ExpandedListArgument(const std::string& arg,
                                                       bool emptyArgs)`

Both return the  `std::vector<std::string>` instead of taking a return vector
reference like `cmSystemTools::ExpandLists` and
`cmSystemTools::ExpandListArgument`.

Motivation
----------

Since C++17 return value optimization is mandatory, so returning a
`std:vector<std::string>` from a function should be (at least) as fast as
passing a return vector reference to the function.

The new methods can replace `cmSystemTools::ExpandLists` and
`cmSystemTools::ExpandListArgument` in many cases, which leads to
shorter and simpler syntax.

E.g. the commonly used pattern
```
  if (const char* value = X->GetProperty("A_KEY_STRING")) {
    std::vector<std::string> valuesList;
    cmSystemTools::ExpandListArgument(value, valuesList);
    for (std::string const& i : valuesList) {
      doSomething(i);
    }
  }
```
becomes
```
  if (const char* value = X->GetProperty("A_KEY_STRING")) {
    for (std::string const& i :
      cmSystemTools::ExpandedListArgument(value)) {
      doSomething(i);
    }
  }
```

1 files changed, 4 insertions, 6 deletions

diff --git a/Source/cmRemoveCommand.cxx b/Source/cmRemoveCommand.cxx
index bb14e687d4..a64ad8c7dd 100644
--- a/Source/cmRemoveCommand.cxx
+++ b/Source/cmRemoveCommand.cxx
@@ -25,15 +25,13 @@ bool cmRemoveCommand::InitialPass(std::vector<std::string> const& args,
   }
 
   // expand the variable
-  std::vector<std::string> varArgsExpanded;
-  cmSystemTools::ExpandListArgument(cacheValue, varArgsExpanded);
+  std::vector<std::string> const varArgsExpanded =
+    cmSystemTools::ExpandedListArgument(cacheValue);
 
   // expand the args
   // check for REMOVE(VAR v1 v2 ... vn)
-  std::vector<std::string> argsExpanded;
-  std::vector<std::string> temp;
-  temp.insert(temp.end(), args.begin() + 1, args.end());
-  cmSystemTools::ExpandList(temp, argsExpanded);
+  std::vector<std::string> const argsExpanded =
+    cmSystemTools::ExpandedLists(args.begin() + 1, args.end());
 
   // now create the new value
   std::string value;