[6.2] Disable surprising lifetime inference of implicit initializers

[atrick]

Non-escapable struct definitions often have inicidental integer fields that are
unrelated to lifetime. Without an explicit initializer, the compiler would infer
these fields to be borrowed by the implicit intializer.

struct CountedSpan: ~Escapable {
  let span: Span<Int>
  let i: Int

  /* infer: @lifetime(copy span, borrow i) init(...) */
}

This was done because

• we always want to infer lifetimes of synthesized code if possible
• inferring a borrow dependence is always conservative

But this was the wrong decision because it inevitabely results in lifetime
diagnostic errors elsewhere in the code that can't be tracked down at the use
site:

let span = CountedSpan(span: span, i: 3) // ERROR: span depends on the lifetime of this value

Instead, force the author of the data type to specify whether the type actually
depends on trivial fields or not. Such as:

struct CountedSpan: ~Escapable {
  let span: Span<Int>
  let i: Int

  @lifetime(copy span) init(...) { ... }
}

This fix enables stricter diagnostics, so we need it in 6.2.

Fixes rdar://152130977 ([nonescapable] confusing diagnostic message when a
synthesized initializer generates dependence on an Int parameter)

(cherry picked from commit 8789a68)

--- CCC ---

Explanation: Disable an inference rule that was unexpected and usually undesirable. See the discussion above.

Scope: This only affects users of the experimental feature: Lifetimes.

Radar/SR Issue: rdar://152130977 ([nonescapable] confusing diagnostic message when a synthesized initializer generates
dependence on an Int parameter)

main PR: #82472

Risk: Low.

This may initially break some code that adopts Lifetimes, but only cases in which the code was not doing what the author
intended. This is a good source break. Failing to make this change now means much worse source breaking change in a
later release.

Testing: Added several source-level unit tests

Reviewer: Meghana Gupta

[atrick]

@swift-ci test