Suppress some cases in `Source/cmGeneratorExpressionNode.cxx` and
`Source/cmUVHandlePtr.h` where a few older compilers require a
user-defined default constructor (with `{}`).
Carry a backtrace on every link item, not just link implementation
items. For now the non-impl items will still have empty backtraces at
runtime, but this will allow us to introduce values over time.
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.
