Tag `swift-6.0.3-RELEASE` cannot be built with Xcode 16.2

[byron-hawkins]

Attempting to build a clean checkout of tag swift-6.0.3-RELEASE with a fresh install of Xcode 16.2 results in a known error. In clang built from any swift-6.0.x-* tag, this compile error is not defined at all, but in clang built on any swift-6.1-*tag, this compile error is defined and implemented. According to the official Xcode wikipedia page, Xcode 16.2 is based on swift-6.0.3-RELEASE, but clearly this is incorrect, because:

• an Xcode 16.2 build of LLVM on any swift-6.0.x-* tag will fail with a compile error about "non-private labels" and cfi_startproc / cfi_endproc pairs
• no swift-6.0.x-* tag contains the code from which clang generates that "non-private labels" error

We conclude that the swift-6.0.3-RELEASE contains a snapshot of LLVM that is in no way relevant to, or coherent with, the default Xcode 16.2 toolchain.

As a workaround, we are using Xcode 16.4 with tag swift-6.1.2-RELEASE, which for our purposes makes a sufficient correspondence between the default Xcode toolchain and the open-source LLVM build.