Improve captured field diagnostics
This patch improves the error message emitted when the capture list
contains an item that is a sub-field of a struct/class/etc....
If the closure capture did not include `weak` at the beginning, the
presence of a period would cause the if-chain to fall through the
identifier checking, resulting in an error message about expecting a
`weak` keyword. Instead, I've opted to accept the period at that stage
of parsing so that we can fall through to a better error message.
For the following code
```
{ [self.field] in ... }
```
instead of emitting
`expected 'weak', 'unowned', or no specifier in capture list`,
we now emit
`fields may only be captured by assigning to a specific name`
with a fix-it that changes the code to
```
{ [ field = self.field ] in ... }
```
