Update swift-syntax package dependency to 603 (#1286)

This updates our package dependency on swift-syntax to `603.0.0-latest`
(replacing `602-0.0-latest`). I also took the opportunity to document
the workaround we use to ensure we're always using the latest
"prerelease" tag.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Package.swift

@@ -109,7 +109,15 @@ let package = Package(
   }(),
 
   dependencies: [
-    .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "602.0.0-latest"),
+    // swift-syntax periodically publishes a new tag with a suffix of the format
+    // "-prerelease-YYYY-MM-DD". We always want to use the most recent tag
+    // associated with a particular Swift version, without needing to hardcode
+    // an exact tag and manually keep it up-to-date. Specifying the suffix
+    // "-latest" on this dependency is a workaround which causes Swift package
+    // manager to use the lexicographically highest-sorted tag with the
+    // specified semantic version, meaning the most recent "prerelease" tag will
+    // always be used.
+    .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "603.0.0-latest"),
   ],
 
   targets: [

Sources/TestingMacros/CMakeLists.txt

@@ -31,7 +31,7 @@ if(SwiftTesting_BuildMacrosAsExecutables)
   set(FETCHCONTENT_BASE_DIR ${CMAKE_BINARY_DIR}/_d)
   FetchContent_Declare(SwiftSyntax
     GIT_REPOSITORY https://github.com/swiftlang/swift-syntax
-    GIT_TAG 340f8400262d494c7c659cd838223990195d7fed) # 602.0.0-prerelease-2025-04-10
+    GIT_TAG 07bf225e198119c23b2b9a0a3432bdb534498873) # 603.0.0-prerelease-2025-08-11
   FetchContent_MakeAvailable(SwiftSyntax)
 endif()