Add support for throwing functions in .memberwise conversions

[coenttb]

Summary

• Adds support for using throwing initializers with .memberwise conversions
• Uses @_disfavoredOverload to maintain backward compatibility
• Implements ThrowingMemberwise struct that properly propagates errors during parsing
• Includes comprehensive tests for both throwing and non-throwing scenarios

Changes

• Extended .memberwise API: Added overload that accepts @escaping (Values) throws -> Struct initializers
• New ThrowingMemberwise struct: Handles throwing initializers while preserving all existing safety checks and memory layout validation
• Comprehensive tests: Added MemberwiseTests.swift with coverage for both success and error cases
• Backward compatibility: Existing non-throwing .memberwise usage remains unchanged

Test Plan

• All existing tests pass (253 tests)
• New tests verify throwing memberwise functionality works correctly
• Non-throwing memberwise still works as expected
• Throwing initializers properly propagate errors during parsing
• Build succeeds without warnings

Implementation Details

The solution uses @_disfavoredOverload on the throwing overload to resolve compiler ambiguity, ensuring the non-throwing version is preferred when both signatures are compatible. The ThrowingMemberwise struct mirrors the original Memberwise implementation but with a throwing apply method that calls try self.initializer(input).