Sema: Fix openType() handling of parameter packs [5.10]

[slavapestov]

5.10 cherry-pick of #70457.

• Description: Fixes some incorrect logic in the code path where we "open" a generic type to obtain a type with type variables. Manifests with various common usages of parameter packs as an assertion failure in an assert build, or various IRGen crashes otherwise.

  Some diagnostics change in a way that is still correct, but makes them less clear. We believe these regressions are acceptable for 5.10. On main, a follow-up PR will fix most of the regressions with a change that's too risky to take for 5.10.

• Origination: Fixes a bug in a new feature, parameter packs.

• Scope of the issue: Seems to impact many users with numerous dupes.

• Radar:

  • rdar://112785081
  • Fixes Compiler crash with for...in statement involving parameter packs #67906
  • Fixes Initialization of a generic structure with a pack parameter using tuple causes compiler crash. #68160
  • Fixes Generic struct using type pack can't store closure using same type pack in property #68369
  • Fixes Parameter Pack Error: extra arguments at positions #2, #3, #4, #5, #6, #7, #8, #9, #10, #11, #12, #13, #14, #15, #16, #17, #18, #19, #20, #21, #22, #23, #24, #25, #26, #27, #28, #29, #30, #31, #32, #33, #34, #35, #36, #37, #38, #39, #40, #41, #42, #43, #44, #45, #46, #47, #48, #49, #50...#65536 #67984

• Tested: Various new test cases reduced from user reports.

• Risk: Low; worst case a regression here should only impact type checking of parameter packs.

• Reviewed by: @xedin

[slavapestov]

@swift-ci Please test