Package.swift fixes for Swift 5.4 release (#797)

* Use Swift 5.4 release of swift-format

* Only set -enable-experimental-distributed for Linux

* Use development snapshot of swift-syntax for Swift 5.5

Author: budde

Package.swift

@@ -11,7 +11,7 @@ import PackageDescription
 var globalSwiftSettings: [SwiftSetting]
 
 var globalConcurrencyFlags: [String] = []
-#if swift(>=5.4)
+#if swift(>=5.4) && os(Linux)
 globalConcurrencyFlags.append(contentsOf: [
     "-Xfrontend", "-enable-experimental-concurrency",
     // "-Xfrontend", "-enable-experimental-distributed",
@@ -296,9 +296,13 @@ var dependencies: [Package.Dependency] = [
     .package(url: "https://github.com/apple/swift-argument-parser", .upToNextMinor(from: "0.3.2")), // not API stable, Apache v2
 ]
 
-#if swift(>=5.4)
+#if swift(>=5.5)
 dependencies.append(
-    .package(name: "SwiftSyntax", url: "https://github.com/apple/swift-syntax.git", .revision("swift-DEVELOPMENT-SNAPSHOT-2021-03-25-a"))
+    .package(name: "SwiftSyntax", url: "https://github.com/apple/swift-syntax.git", .revision("swift-DEVELOPMENT-SNAPSHOT-2021-05-18-a"))
+)
+#elseif swift(>=5.4)
+dependencies.append(
+    .package(name: "SwiftSyntax", url: "https://github.com/apple/swift-syntax.git", .exact("0.50400.0"))
 )
 #elseif swift(>=5.3)
 dependencies.append(