Warn about the use of internal bridging headers without library evolution

It's very, very easy to make a mistake that will cause broken
serialized modules. Until that's no longer true, at least tell folks
that they are heading into uncharted waters, as we do with
`@_implementationOnly` imports.

Author: DougGregor

include/swift/AST/DiagnosticsFrontend.def

@@ -609,6 +609,9 @@ ERROR(no_swift_sources_with_embedded,none,
 ERROR(package_cmo_requires_library_evolution, none,
         "Library evolution must be enabled for Package CMO", ())
 
+WARNING(internal_bridging_header_without_library_evolution,none,
+        "using internal bridging headers without library evolution can cause instability", ())
+
 ERROR(experimental_not_supported_in_production,none,
       "experimental feature '%0' cannot be enabled in production compiler",
       (StringRef))

lib/Frontend/CompilerInvocation.cpp

@@ -2129,6 +2129,13 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
     Opts.BridgingHeaderIsInternal = importAsInternal;
   }
 
+  // Until we have some checking in place, internal bridging headers are a
+  // bit unsafe without library evolution.
+  if (Opts.BridgingHeaderIsInternal && !FrontendOpts.EnableLibraryEvolution) {
+    Diags.diagnose(SourceLoc(),
+                   diag::internal_bridging_header_without_library_evolution);
+  }
+
   Opts.DisableSwiftBridgeAttr |= Args.hasArg(OPT_disable_swift_bridge_attr);
 
   Opts.DisableOverlayModules |= Args.hasArg(OPT_emit_imported_modules);

test/ClangImporter/InternalBridgingHeader/library_evolution.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -internal-import-bridging-header %S/../Inputs/c-bridging-header.h -sdk %clang-importer-sdk %s 2>&1 | %FileCheck -check-prefix NONRESILIENT %s
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -internal-import-bridging-header %S/../Inputs/c-bridging-header.h -sdk %clang-importer-sdk %s -enable-library-evolution 2>&1 | %FileCheck -check-prefix EVOLUTION %s
+
+// NONRESILIENT: warning: using internal bridging headers without library evolution can cause instability
+
+// EVOLUTION-NOT: internal bridging head
+
+@available(*, deprecated)
+func f() { }
+
+func g(){
+  f() // make sure we emit something
+}