The following diagnostics have been added:
1. implicit capture of inout arguments by closure literals that may escape (
not mark by @noescape) is now invalid. This also includes implicit capture
of `self`.
2. nested functions with inout captures cannot be used as arguments not marked
@noescape.
3. nested function with inout captures cannot be a return value.
This change eliminates the need for the shadowing mechanism created for inouts.
