[ModuleInterface] Implement silencing errors on rebuild with a new engine

lib/Frontend/ModuleInterfaceBuilder.cpp

@@ -320,11 +320,19 @@ bool ImplicitModuleInterfaceBuilder::buildSwiftModuleInternal(
       llvm::RestorePrettyStackState(savedInnerPrettyStackState);
     };
 
+    Optional<DiagnosticEngine> localDiags;
+    DiagnosticEngine *rebuildDiags = diags;
+    if (silenceInterfaceDiagnostics) {
+      // To silence diagnostics, use a local temporary engine.
+      localDiags.emplace(sourceMgr);
+      rebuildDiags = &*localDiags;
+    }
+
     SubError = (bool)subASTDelegate.runInSubCompilerInstance(
         moduleName, interfacePath, OutPath, diagnosticLoc,
         [&](SubCompilerInstanceInfo &info) {
           auto EBuilder = ExplicitModuleInterfaceBuilder(
-              *info.Instance, diags, sourceMgr, moduleCachePath, backupInterfaceDir,
+              *info.Instance, rebuildDiags, sourceMgr, moduleCachePath, backupInterfaceDir,
               prebuiltCachePath, ABIDescriptorPath, extraDependencies, diagnosticLoc,
               dependencyTracker);
           return EBuilder.buildSwiftModuleFromInterface(

test/Index/index_system_modules_swiftinterfaces.swift

@@ -66,6 +66,7 @@
 // RUN: c-index-test core -print-record %t/idx | %FileCheck -check-prefix=BROKEN-RECORD %s
 // BROKEN-RECORD-NOT: function/Swift | systemFunc()
 
+/// Subsequent builds won't attempt to index the broken swiftinterface again
 // RUN: %target-swift-frontend -typecheck -parse-stdlib \
 // RUN:     -index-system-modules \
 // RUN:     -index-store-path %t/idx \
@@ -78,6 +79,20 @@
 // RUN:     2>&1 | %FileCheck -check-prefix=BROKEN-BUILD-2 --allow-empty %s
 // BROKEN-BUILD-2-NOT: indexing system module
 
+/// Local errors should be preserved even when indexing against a broken swiftinterface
+// RUN: %empty-directory(%t/idx)
+// RUN: %empty-directory(%t/modulecache)
+// RUN: not %target-swift-frontend -typecheck -parse-stdlib \
+// RUN:     -index-system-modules \
+// RUN:     -index-store-path %t/idx \
+// RUN:     -index-ignore-stdlib \
+// RUN:     -sdk %t/SDK \
+// RUN:     -Fsystem %t/SDK/Frameworks \
+// RUN:     -module-cache-path %t/modulecache \
+// RUN:     %t/ClientWithError.swift 2> %t/client-with-error.err
+// RUN: cat %t/client-with-error.err | %FileCheck -check-prefix=WITH-ERROR %s
+// WITH-ERROR: cannot convert return expression of type 'U' to return type 'T'
+
 //--- SecretModule.swift
 public struct SecretType {}
 
@@ -92,3 +107,12 @@ func leakyFunc(_ a: SecretType) { }
 import SystemModule
 
 public func clientFunc() {}
+
+//--- ClientWithError.swift
+
+import SystemModule
+public func clientFunc() {}
+
+struct T {}
+struct U {}
+func f() -> T { return U() }