Prepare for strict enforcement of conditionally Escapable
The only intended functional change is that a diagnostic message is split into
an error and a note:
error: cannot copy the lifetime of an Escapable type
note: use '@_lifetime(borrow arg)' instead
Reorganize the LifetimeDependenceChecker to support adding the functionality
required for strict enforcement.
Adds parameter index to ParamInfo.
Adds a few (temporarily unused) helper methods.
