[6.2] Sema: Fix the insertion location for conformances attributes

[AnthonyLatsis]

• Explanation:

  We were using ProtocolNameLoc for the insertion location, which is not necessarily the start location of the TypeRepr associated with the conformance:

  warning: conformance of 'S' to 'P' should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances' [isolated_conformance_will_become_nonisolated] [#IsolatedConformances]
  @MainActor struct S: @unsafe Nested.P {}
                                      ^
                                      nonisolated
  

  This fix is important for error-free automatic migration to StrictMemorySafety and InferIsolatedConformances.

• Scope: Diagnostics QoI for fix-its that insert a conformance attribute.

• Issues: rdar://132570262

• Original PRs: Sema: Fix the insertion location for conformances attributes #82452

• Risk: Low

• Testing: Smoke test. Added regression tests.

• Reviewers: @DougGregor

[AnthonyLatsis]

@swift-ci please smoke test macOS

[AnthonyLatsis]

@swift-ci please test

[AnthonyLatsis]

@swift-ci please test macOS