Move import warning silencing logic

Move the warning suppression code from the
frontend and into import resolution. Parsing
already has its own logic for silencing warnings.

Author: hamishknight

lib/Frontend/Frontend.cpp

@@ -929,10 +929,6 @@ void CompilerInstance::parseAndTypeCheckMainFileUpTo(
   SourceFile &MainFile =
       MainModule->getMainSourceFile(Invocation.getSourceFileKind());
 
-  auto &Diags = MainFile.getASTContext().Diags;
-  auto DidSuppressWarnings = Diags.getSuppressWarnings();
-  Diags.setSuppressWarnings(DidSuppressWarnings || !mainIsPrimary);
-
   // For a primary, perform type checking if needed. Otherwise, just do import
   // resolution.
   if (mainIsPrimary && LimitStage >= SourceFile::TypeChecked) {
@@ -948,8 +944,6 @@ void CompilerInstance::parseAndTypeCheckMainFileUpTo(
     parseSourceFileSIL(MainFile, &SILContext);
   }
 
-  Diags.setSuppressWarnings(DidSuppressWarnings);
-
   if (mainIsPrimary && !Context->hadError() &&
       Invocation.getFrontendOptions().DebuggerTestingTransform) {
     performDebuggerTestingTransform(MainFile);

lib/Sema/ImportResolution.cpp

@@ -282,6 +282,17 @@ void swift::performImportResolution(SourceFile &SF) {
 
   FrontendStatsTracer tracer(SF.getASTContext().Stats,
                              "Import resolution");
+
+  // If we're silencing parsing warnings, then also silence import warnings.
+  // This is necessary for secondary files as they can be parsed and have their
+  // imports resolved multiple times.
+  auto &diags = SF.getASTContext().Diags;
+  auto didSuppressWarnings = diags.getSuppressWarnings();
+  auto shouldSuppress = SF.getParsingOptions().contains(
+      SourceFile::ParsingFlags::SuppressWarnings);
+  diags.setSuppressWarnings(didSuppressWarnings || shouldSuppress);
+  SWIFT_DEFER { diags.setSuppressWarnings(didSuppressWarnings); };
+
   ImportResolver resolver(SF);
 
   // Resolve each import declaration.