Implement SwiftFixIt — a library for deserializing diagnostics and applying fix-its (#8585)

### Motivation:

This is part of the [migration
tooling](swiftlang/swift-evolution#2673)
initiative for Swift features.

### Modifications:

For now, this API accepts a collection of `.dia` files, deserializes
them, and applies fix-its in place. Eventually, it is expected to serve
the upcoming `swift fixit` and `swift migrate` subcommands and support
various filtering options, as well as a stateful, interactive workflow
similar to `git add --patch`.

Author: AnthonyLatsis

Package.swift

@@ -304,6 +304,21 @@ let package = Package(
             ]
         ),
 
+        .target(
+            /** API for deserializing diagnostics and applying fix-its */
+            name: "SwiftFixIt",
+            dependencies: [
+                "Basics",
+                .product(name: "TSCBasic", package: "swift-tools-support-core"),
+            ] + swiftSyntaxDependencies(
+                ["SwiftDiagnostics", "SwiftIDEUtils", "SwiftParser", "SwiftSyntax"]
+            ),
+            exclude: ["CMakeLists.txt"],
+            swiftSettings: commonExperimentalFeatures + [
+                .unsafeFlags(["-static"]),
+            ]
+        ),
+
         // MARK: Project Model
 
         .target(
@@ -916,6 +931,10 @@ let package = Package(
             dependencies: ["SourceControl", "_InternalTestSupport"],
             exclude: ["Inputs/TestRepo.tgz"]
         ),
+        .testTarget(
+            name: "SwiftFixItTests",
+            dependencies: ["SwiftFixIt", "_InternalTestSupport"]
+        ),
         .testTarget(
             name: "XCBuildSupportTests",
             dependencies: ["XCBuildSupport", "_InternalTestSupport", "_InternalBuildTestSupport"],

Sources/SwiftFixIt/CMakeLists.txt

@@ -0,0 +1,24 @@
+# This source file is part of the Swift open source project
+#
+# Copyright (c) 2025 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+add_library(SwiftFixIt STATIC
+  SwiftFixIt.swift)
+target_link_libraries(SwiftFixIt PUBLIC
+  Basics
+
+  SwiftSyntax::SwiftDiagnostics
+  SwiftSyntax::SwiftIDEUtils
+  SwiftSyntax::SwiftParser
+  SwiftSyntax::SwiftSyntax
+
+  TSCBasic
+  TSCUtility
+)
+
+set_target_properties(SwiftFixIt PROPERTIES
+  INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})