[JSON] 5.7 compatibility

* Add SE-0370 methods
* Return concrete type instead of existential

(cherry picked from commit 7d41485)

Author: rintaro

Sources/SwiftCompilerPluginMessageHandling/JSON/JSONDecoding.swift

@@ -1173,7 +1173,7 @@ extension JSONDecoding.UnkeyedContainer: UnkeyedDecodingContainer {
   }
 
   @inline(__always)
-  mutating func _getOrThrow() throws -> (index: any CodingKey, value: JSONMapValue) {
+  mutating func _getOrThrow() throws -> (index: _CodingKey, value: JSONMapValue) {
     let idx = currentIndex
     guard !isAtEnd else {
       throw DecodingError.valueNotFound(
@@ -1314,3 +1314,40 @@ extension JSONDecoding.UnkeyedContainer: UnkeyedDecodingContainer {
     self._currMapIdx = array.startIndex
   }
 }
+
+// Compatibility shim for SE-0370
+#if swift(<5.8)
+extension UnsafeMutableBufferPointer {
+  fileprivate func initialize(
+    fromContentsOf source: some Collection<Element>
+  ) -> Index {
+    let count = source.withContiguousStorageIfAvailable {
+      guard let sourceAddress = $0.baseAddress, !$0.isEmpty else {
+        return 0
+      }
+      precondition(
+        $0.count <= self.count,
+        "buffer cannot contain every element from source."
+      )
+      baseAddress?.initialize(from: sourceAddress, count: $0.count)
+      return $0.count
+    }
+    if let count {
+      return startIndex.advanced(by: count)
+    }
+
+    var (iterator, copied) = self.initialize(from: source)
+    precondition(
+      iterator.next() == nil,
+      "buffer cannot contain every element from source."
+    )
+    return startIndex.advanced(by: copied)
+  }
+
+  fileprivate func initializeElement(at index: Index, to value: Element) {
+    precondition(startIndex <= index && index < endIndex)
+    let p = baseAddress!.advanced(by: index)
+    p.initialize(to: value)
+  }
+}
+#endif