[6.0][immediate] Load Foundation early enough for bridging

[benlangmuir]

• Explanation: Some bridged Foundation types require Foundation to be loaded in the process before we ever reach certain code paths. Swift scripts using such types were crashing, because they were loading Foundation too late. This change unconditionally loads Foundation early in the frontend process when in scripting mode to workaround this issue.
• Scope: Only affects running swift scripts, and only on Darwin. For scripts that import Foundation the only difference is we dlopen it earlier. For scripts that don't use Foundation they will now dlopen Foundation, because it's too early in the process to detect whether it's needed. The risk to such scripts is mitigated because failure to load Foundation is only a warning not an error.
• Issues: rdar://129528115
• Original PRs: [immediate] Load Foundation early enough for bridging #75594
• Risk: The risk of regression from this change is low and scope is limited. The primary risk for this change is that we could make future changes to the compiler that introduce swift bridging calls earlier in the process and cause this bug to return. That should be caught by CI running tests with --param use_os_stdlib, but that isn't in PR test configs so it may not be caught immediately. We have multiple options to fix this bug more thoroughly for the future though either in the runtime/Foundation or in the immediate mode JIT, but they are all much more involved changes.
• Testing: Tested manually on macOS 15 beta and macOS 13.6. On 13.6, also checked multiple configurations (arm64, arm64 with SIP disabled, x86_64). There's a lit test included that will run with --param use_os_stdlib.
• Reviewers: @mikeash

[benlangmuir]

@swift-ci please test

[fredriss]

Given this obviously only affects immediate mode, I'm approving it.