swift234

Thread constraint locators through opening of generic types.

Locators that refer to opened type parameters now carry information
about the source location where we needed to open the type, so that
(for example) we can trace an opened type parameter back to the
location it was opened. As part of this, eliminate the "rootExpr"
fallback, because we're threading constraint locators everywhere.

This is infrastructural, and should be NFC.

Swift SVN r21919