Fix build failure when building with pre-6.2 toolchain due to unrecognized unsafe keyword

[stmontgomery]

This fixes another bit of fallout from #1069 when building this project's test targets with a 6.1 (or any pre-6.2) toolchain.

The unsafe keyword was introduced in 6.2 as part of SE-0458: Opt-in Strict Memory Safety Checking. Older toolchains are not aware of it, so the fix is to avoid emitting expressions involving that keyword when the macro plugin has been built using an older toolchain.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test 6.1

[stmontgomery]

@swift-ci please test

[grynspan]

When building from source locally with the release 6.1 toolchain, I'm seeing the compiler continue to complain that "Attribute requires '-enable-experimental-feature AllowUnsafeAttribute'" on __requiringUnsafe(). So I think we need to guard the @unsafe attribute there with:

#if hasFeature(AllowUnsafeAttribute)
@unsafe
#endif

[stmontgomery]

When building from source locally with the release 6.1 toolchain, I'm seeing the compiler continue to complain that "Attribute requires '-enable-experimental-feature AllowUnsafeAttribute'" on __requiringUnsafe()

That should be fixed by #1084. Or, are you saying your local checkout has that fix but you're still seeing this error? With that fix plus this PR, I am able to build using the 6.1 release toolchain successfully.

[grynspan]

I have main checked out and I see the diagnostic with:

> swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
> Target: arm64-apple-macosx15.0

I see the diagnostic on this branch too.

Did a clean build and it's gone. Disregard!

[stmontgomery]

The macOS and Linux CI failures are expected to be resolved by swiftlang/swift#80830, but the release of a new main snapshot toolchain is currently blocked for unrelated reasons. Proceeding to merge.