[cristim]

Summary

Make Apple's typed-allocation Clang flags optional in the OpenAttributeGraph SwiftPM manifest. Native builds keep the current behavior by default. A Darwin cross build using a Clang without -ftyped-memory-operations or -ftyped-cxx-new-delete can set OPENATTRIBUTEGRAPH_TYPED_MEMORY_OPERATIONS=0.

Verification

• A target-aware arm64 macOS OpenSwiftUI_SPI build with local OpenAttributeGraph and OpenSwiftUI forks initially stopped at clang: error: unknown argument: '-ftyped-memory-operations' and '-ftyped-cxx-new-delete'.
• With the new manifest option set to 0 in both forks, the build passed those Clang errors and reached the next source/API gap in OpenSwiftUI_SPI (NSAttributedString initWithFormat:options:locale:arguments:).
• git diff --check passed.

This PR changes only the opt-out path for cross toolchains; the default flags remain enabled for native Apple builds.

[cristim]

Added a second cross-build setting in commit 4086eb1: OPENATTRIBUTEGRAPH_SWIFT_CHECKOUT_PATH selects an existing Swift source checkout for the C/C++ runtime headers. The default remains Checkouts/swift. The local Darling cross build had an empty default checkout; setting this to the local Swift 6.3.3 source removes the swift/Runtime/Metadata.h missing-header error and advances compilation to the next SDK/toolchain issues. This has been verified through the target-aware OpenSwiftUI dependency build; it does not claim a complete OpenAttributeGraph or SwiftUI runtime.