[PlaygroundRuntime] Migrated the experimental PlaygroundRuntime framework to Swift 4.2.

This is for <rdar://problem/45676453>.

Author: cwakamo

Experimental/PlaygroundRuntime/PlaygroundRuntime.xcodeproj/project.pbxproj

@@ -106,7 +106,7 @@
 				TargetAttributes = {
 					5EB7828A1FAE6B6F00ADE84E = {
 						CreatedOnToolsVersion = 9.1;
-						LastSwiftMigration = 0910;
+						LastSwiftMigration = "";
 						ProvisioningStyle = Automatic;
 					};
 				};
@@ -207,6 +207,7 @@
 				SDKROOT = macosx;
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_VERSION = 4.2;
 				TVOS_DEPLOYMENT_TARGET = 10.0;
 				VERSIONING_SYSTEM = "apple-generic";
 				VERSION_INFO_PREFIX = "";
@@ -261,6 +262,7 @@
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = macosx;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
+				SWIFT_VERSION = 4.2;
 				TVOS_DEPLOYMENT_TARGET = 10.0;
 				VALIDATE_PRODUCT = YES;
 				VERSIONING_SYSTEM = "apple-generic";
@@ -288,7 +290,6 @@
 				SUPPORTED_PLATFORMS = "macosx iphonesimulator iphoneos appletvsimulator appletvos";
 				SWIFT_INSTALL_OBJC_HEADER = NO;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};
@@ -311,7 +312,6 @@
 				SKIP_INSTALL = YES;
 				SUPPORTED_PLATFORMS = "macosx iphonesimulator iphoneos appletvsimulator appletvos";
 				SWIFT_INSTALL_OBJC_HEADER = NO;
-				SWIFT_VERSION = 4.0;
 			};
 			name = Release;
 		};