[6.0] Ensure we are using mapped SIL type for switch_enum case and not the original lowered one

[clackary]

• Explanation:
  Cherrypicks the fix for the assertion failure from Another (srcAddr->getType() == destAddr->getType()), function createCopyAddr assertion failure. #73018 into 6.0 release toolchains.
• Scope:
  Looks like this mainly affects autodiff code.
• Issues:
  Another (srcAddr->getType() == destAddr->getType()), function createCopyAddr assertion failure. #73018
• Original PRs:
  Ensure we are using mapped SIL type for switch_enum case and not the original lowered one #73385
• Risk:
  Low, the original fix has landed and stuck on main for months.
• Testing:
  A test case is included. No additional testing needed.
• Reviewers:
  @asl
  @aschwaighofer
  @slavapestov

[clackary]

@shahmishal I noticed there's a 6.0.2 branch. Not knowing how close the target release date is for that, would this have a chance of being included in 6.0.2 release?

We'd like to adopt release toolchains, but this is one blocker that would prevent us from doing so.

[asl]

@clackary There were some other fixes to LoadableByAddress, notably:
#72619 and #72617

You might probably want to pick them as well (do not know if they are in 6.0 release branch, I'd assume – no).

[clackary]

Looks like they weren't, so I've opened a few PRs for those also:
#77110
#77111

[DougGregor]

@swift-ci please test

[DougGregor]

This addresses an assertion and has been thoroughly tested on main. We'll take it