Add an experimental option TransferringArgsAndResults.

Just splitting off a larger commit to make this easier to review.

Author: gottesmm

include/swift/Basic/Features.def

@@ -281,6 +281,9 @@ EXPERIMENTAL_FEATURE(FixedArrays, true)
 // Group Main Actor Isolation Errors by Scope
 EXPERIMENTAL_FEATURE(GroupActorErrors, true)
 
+// Allow for the 'transferring' keyword to be applied to arguments and results.
+EXPERIMENTAL_FEATURE(TransferringArgsAndResults, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3870,6 +3870,8 @@ static bool usesFeatureExtractConstantsFromMembers(Decl *decl) {
 
 static bool usesFeatureBitwiseCopyable(Decl *decl) { return false; }
 
+static bool usesFeatureTransferringArgsAndResults(Decl *decl) { return false; }
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {

lib/ASTGen/Sources/ASTGen/SourceFile.swift

@@ -55,6 +55,7 @@ extension Parser.ExperimentalFeatures {
     mapFeature(.TypedThrows, to: .typedThrows)
     mapFeature(.DoExpressions, to: .doExpressions)
     mapFeature(.NonescapableTypes, to: .nonescapableTypes)
+    mapFeature(.TransferringArgsAndResults, to: .transferringArgsAndResults)
   }
 }