AST: Add convenience for limiting diags to warnings in swiftinterfaces.

Sometimes it's useful to be more lenient when type checking swiftinterfaces
since restrictions that could be dropped in the future will manifest in
resilient libraries being incompatible with older compilers otherwise.

Author: tshortli

include/swift/AST/DiagnosticEngine.h

@@ -564,6 +564,15 @@ namespace swift {
     /// until the next major language version.
     InFlightDiagnostic &warnUntilSwiftVersion(unsigned majorVersion);
 
+    /// Limit the diagnostic behavior to warning if the context is a
+    /// swiftinterface.
+    ///
+    /// This is useful for diagnostics for restrictions that may be lifted by a
+    /// future version of the compiler. In such cases, it may be helpful to
+    /// avoid failing to build a module from its interface if the interface was
+    /// emitted using a compiler that no longer has the restriction.
+    InFlightDiagnostic &warnInSwiftInterface(const DeclContext *context);
+
     /// Conditionally limit the diagnostic behavior to warning until
     /// the specified version.  If the condition is false, no limit is
     /// imposed, meaning (presumably) it is treated as an error.

lib/AST/DiagnosticEngine.cpp

@@ -334,6 +334,16 @@ InFlightDiagnostic::warnUntilSwiftVersion(unsigned majorVersion) {
   return *this;
 }
 
+InFlightDiagnostic &
+InFlightDiagnostic::warnInSwiftInterface(const DeclContext *context) {
+  auto sourceFile = context->getParentSourceFile();
+  if (sourceFile && sourceFile->Kind == SourceFileKind::Interface) {
+    return limitBehavior(DiagnosticBehavior::Warning);
+  }
+
+  return *this;
+}
+
 InFlightDiagnostic &
 InFlightDiagnostic::wrapIn(const Diagnostic &wrapper) {
   // Save current active diagnostic into WrappedDiagnostics, ignoring state