cmLinkItem: Convert to a "sum type" over a string and target pointer

Avoid exposing the item name implicitly as std::string.  When the item
is a target, avoid storing a second copy of its name.

Most link item construction is paired with calls to `FindTargetToLink`
to get the possible target pointer.  Rename these methods to
`ResolveLinkItem` and refactor them to construct the entire item.

Source/cmGeneratorTarget.h

@@ -357,7 +357,7 @@ public:
                                           cmOptionalLinkImplementation& impl,
                                           const cmGeneratorTarget* head) const;
 
-  cmGeneratorTarget* FindTargetToLink(std::string const& name) const;
+  cmLinkItem ResolveLinkItem(std::string const& name) const;
 
   // Compute the set of languages compiled by the target.  This is
   // computed every time it is called because the languages can change