Previously in the case of a constructor like `A<Int>(value: 1)`
`Fn->getLoc()` returned the location of `>(value: 1)` while the actual
location we're looking for is correctly the start of `A<Int>(value: 1)`.
This adjusts the location we're looking up to use the start location of
the constructor instead.
Fixes: https://github.com/apple/swift/issues/54532
Extensions redeclare all generic parameters of their extended type to add their additional restrictions. There are two issues with this model for indexing:
- The generic paramter declarations of the extension are implicit so we wouldn't report them in the index. Any usage of the generic param in the extension references this implicit declaration so we don't include it in the index either.
- The implicit re-declarations have their own USRs so any usage of a generic parameter inside an extension would use a different USR than declaration of the param in the extended type.
To fix these issues, we replace the reference to the implicit generic parameter defined in the extension by a reference to the generic paramter defined in the extended type.
