SILGen: Add runtime test for Error to NSError conversion

Ensure that a function manipulating an existential Error still supports
NSError types with or without the fast-path optimization.

Author: xymus

test/SILGen/objc_error_fast_path.swift

@@ -0,0 +1,55 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t --leading-lines
+// REQUIRES: objc_interop
+// REQUIRES: executable_test
+
+/// Optimization active: public import
+// RUN: %target-build-swift-dylib(%t/%target-library-name(Lib)) \
+// RUN:   %t/optimized.swift -module-name Lib \
+// RUN:   -emit-module -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -swift-version 5 -enable-library-evolution -O
+// RUN: %target-build-swift %t/main.swift -o %t/main -I%t -lLib -L%t -O
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main | %FileCheck %s
+
+/// Ensure the client has the optimization we're testing here.
+// RUN: %target-swift-frontend -typecheck -emit-silgen %t/Lib.swiftmodule > %t/Lib.sil
+// RUN: %FileCheck --check-prefix CHECK-OPTIMIZED --input-file %t/Lib.sil %s
+// CHECK-OPTIMIZED: _getErrorEmbeddedNSError
+
+/// No optimization: internal import from resilient module
+// RUN: %target-build-swift-dylib(%t/%target-library-name(Lib)) \
+// RUN:   %t/non-optimized.swift -module-name Lib \
+// RUN:   -emit-module -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -swift-version 5 -enable-library-evolution -O
+// RUN: %target-build-swift %t/main.swift -o %t/main -I%t -lLib -L%t -O
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main | %FileCheck %s
+
+/// Ensure the client doesn't have the optimization we're testing here.
+// RUN: %target-swift-frontend -typecheck -emit-silgen %t/main.swift -I%t -O > %t/Lib.sil
+// RUN: %FileCheck --check-prefix CHECK-NOT-OPTIMIZED --input-file %t/Lib.sil %s
+// CHECK-NOT-OPTIMIZED-NOT: _getErrorEmbeddedNSError
+
+//--- optimized.swift
+public import Foundation
+
+@inlinable public func foo<E: Error>(e: E) -> Error {
+    return e
+}
+
+//--- non-optimized.swift
+internal import Foundation
+
+@inlinable public func foo<E: Error>(e: E) -> Error {
+    return e
+}
+
+//--- main.swift
+import Lib
+import Foundation
+
+let err: NSError = NSError(domain: "Not found", code: 404)
+let errOut: Error = foo(e: err)
+print(errOut.localizedDescription)
+// CHECK: Not found error 404.