[Diagnostics] Add InFlightDiagnostic::limitBehaviorUntilSwiftVersion to

downgrade or suppress errors until a specified language version.

Author: hborla

include/swift/AST/DiagnosticEngine.h

@@ -575,6 +575,14 @@ namespace swift {
       return limitBehavior(limit);
     }
 
+    /// Limit the diagnostic behavior to \c limit until the specified
+    /// version.
+    ///
+    /// This helps stage in fixes for stricter diagnostics as warnings
+    /// until the next major language version.
+    InFlightDiagnostic &limitBehaviorUntilSwiftVersion(
+        DiagnosticBehavior limit, unsigned majorVersion);
+
     /// Limit the diagnostic behavior to warning until the specified version.
     ///
     /// This helps stage in fixes for stricter diagnostics as warnings

lib/AST/DiagnosticEngine.cpp

@@ -328,9 +328,10 @@ InFlightDiagnostic::limitBehavior(DiagnosticBehavior limit) {
 }
 
 InFlightDiagnostic &
-InFlightDiagnostic::warnUntilSwiftVersion(unsigned majorVersion) {
+InFlightDiagnostic::limitBehaviorUntilSwiftVersion(
+    DiagnosticBehavior limit, unsigned majorVersion) {
   if (!Engine->languageVersion.isVersionAtLeast(majorVersion)) {
-    limitBehavior(DiagnosticBehavior::Warning)
+    limitBehavior(limit)
       .wrapIn(diag::error_in_future_swift_version, majorVersion);
   }
 
@@ -343,6 +344,12 @@ InFlightDiagnostic::warnUntilSwiftVersion(unsigned majorVersion) {
   return *this;
 }
 
+InFlightDiagnostic &
+InFlightDiagnostic::warnUntilSwiftVersion(unsigned majorVersion) {
+  return limitBehaviorUntilSwiftVersion(DiagnosticBehavior::Warning,
+                                        majorVersion);
+}
+
 InFlightDiagnostic &
 InFlightDiagnostic::warnInSwiftInterface(const DeclContext *context) {
   auto sourceFile = context->getParentSourceFile();