…lchain. (#7901)

… compiler and linker (#7907)

- Explanation:

Fixes a bug where keeping them together broke Symbol Graph Extract tool
because it cannot handle `-Xlinker` flags.

- Main Branch PR:
#7903

- Resolves: rdar://134406349

- Risk: Low (This is technically an NFC since it only re-arranging how
to the flags are propagated but doesn't change them).

- Reviewed By: @MaxDesiatov 

- Testing: New tests were added to the test suite.

…rather than as a package dependency. (#7910)

**Explanation:** Changes package templates to use Swift Testing by
default instead of XCTest and removes package dependencies on the
swift-testing package.
**Scope:** `swift package init`, `swift package add-target`
**Issue:** [N/A](rdar://128272585)
**Original PR:**
#7872
**Risk:** Moderate
**Testing:** Original PR tested with toolchain builds.
**Reviewer:** @bnbarham @briancroom @xedin

`canImport(Testing)` currently fails on the full toolchain test jobs due
to various issues when building for both x86 and arm64. Temporarily
remove this check until we can fix the underlying problems there.

…arget… (#7921)

- Explanation:

Fixes a bug where swift and linker flags for swift-testing were injected
into `extraFlags` of a toolchain that was targeting WASM but used macOS
to build.

- Main Branch PR:
#7920

- Resolves:
#7919
- Resolves: rdar://134714404

- Risk: Low (only affects cross-compilation to WASM).

- Reviewed By: @bnbarham 

- Testing: Manual testing since this is not currently possible to test
automatically.

Cherry-pick of #7969.

**Explanation**: New versions of Darwin OSes are not available as
supported platforms in package manifest APIs.
**Scope**: isolated to `SupportedPlatforms.swift` in
`PackageDescription` public API.
**Risk**: low, no existing code was changed, new constants were defined
in the same way as they were defined for previous versions.
**Testing**: manually verified new versions are available.
**Issue**: rdar://136209148
**Reviewer**: @plemarquand

**Explanation:** Remove `@MainActor` annotations from corelibs-xctest
glue functions that are no longer needed.
**Scope:** corelibs-xctest glue code
**Issue:** [N/A](rdar://130066460)
**Original PR:**
#7967
**Risk:** Unknown
**Testing:** Existing CI jobs
**Reviewer:** @bnbarham @briancroom @DougGregor @shahmishal

Properly handle swift-testing installations in CommandLineTools.

Resolves rdar://136424541.

This PR checks if XCTest is available before invoking XCTest-based tests
on Darwin. There are three possible outcomes:

1. If XCTest is available, we will run XCTest-based tests (as we have
historically.)
2. If XCTest is not available and the user explicitly passed
`--enable-xctest`, we will attempt to run XCTest-based tests, but in
general this code path will continue to fail as `swift test` has
historically done when XCTest is not available.
3. If XCTest is not available and the user did not pass
`--enable-xctest`, we skip running any XCTest logic.

On Linux/Windows/etc., XCTest is always present via
swift-corelibs-xctest and so this change has no practical effect there.
On Darwin, XCTest may be missing if the user has installed the Xcode
Command Line Tools, but not the full Xcode IDE. XCTest is not included
with the Xcode Command Line Tools package.

The purpose of this change is to allow running `swift test` when XCTest
is unavailable but Swift Testing _is_ available.

### Motivation:
Running `swift package --version` indicate it is in development mode,
while `swift --release` does not.

```
❯ swift --version  
Apple Swift version 6.0.1 (swift-6.0.1-RELEASE)
Target: arm64-apple-macosx15.0

❯ swift package --version
Swift Package Manager - Swift 6.0.1-dev
```
### Modifications:

Toggling `isDevelopment` boolean to `false` in `SwiftVersion` struct
extension.

### Result:
```
❯ swift build --show-bin-path
/Users/bkhouri/Documents/git/public/swiftlang/swift-package-manager/.build/arm64-apple-macosx/debug

❯ /Users/bkhouri/Documents/git/public/swiftlang/swift-package-manager/.build/arm64-apple-macosx/debug/swift-package --version
Swift Package Manager - Swift 6.0.2
```