[6.1] Assign fallback discriminators within top-level closures.

[DougGregor]

• Explanation: We recently enabled some checking in the non-asserts compiler to ensure that all closures and local entities get discriminators, the lack of which can cause miscompiles and compiler crashes. We found a case where we were never correctly assigning discriminators, which relates to uses of macros at the top level of code where some of the macro arguments involve closures. Assign discriminators in those cases.
• Scope: Prevents a compiler assertion/crash by assigning closure/macro discriminators for closures that appear at the top level.
• Original PR: Assign fallback discriminators within top-level closures. #78565
• Issue: rdar://142425569
• Risk: Very very low. We're assigning a discriminator in a place that otherwise would have any, and these aren't ABI-stable.
• Testing: Updated tests.

[DougGregor]

@swift-ci please test