This translates the rules for @preconcurrency import from SE-0337 into the
region isolation world. Specifically if a module is compiled without strict
concurrency checking and imported with @preconcurrency:
1. All types from that module that are implicitly non-Sendable have diagnostics
suppressed in swift 5 and swift 6.
2. All types from that module that are explicitly non-Sendable emit warnings in
both swift 5 and swift 6.
rdar://126804052
