swift234

Add MetadataSource classes

The compiler is generally free to not include pointers to metadata in
heap boxes, which are used for closure captures, if it knows you can get
to metadata through some other path. These MetadataSource classes will
describe a sequence of steps to get to metadata at runtime.

In the short term, this will be useful for describing the layout of
function/closure capture contexts, which can vary depending on what is
captured.