The main fixes here are:
- we weren't looking through open-existentials in the l-value
- we weren't handling mutating gets correctly unless CSApply wrapped
the base in an InOutExpr, which seems to be multifile-sensitive
- we were missing diagnostics in some cases involving subscripts
A better fix would be to re-introduce LValueAccessKind, but I wanted
a workable short-term fix that I could try to get into 5.1.
Fixes rdar://49482742, which is specific to the lazy-getter problem.
This makes diagnostics more verbose and accurate, because
it's possible to distinguish how many parameters there are
based on the message itself.
Also there are multiple diagnostic messages in a format of
`<descriptive-kind> <decl-name> ...` that get printed as
e.g. `subscript 'subscript'` if empty labels are omitted.
