Improve diagnostic for broken module interfaces

Currently, when a swiftinterface file fails to load, we emit the specific diagnostics for the failures, followed by a generic “failed to load module ‘Foo’” message. This PR improves that final diagnostic, particularly when the cause may be that the interface was emitted by a newer compiler using backwards-incompatible syntax.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -61,8 +61,8 @@ static void printToolVersionAndFlagsComment(raw_ostream &out,
                                             ModuleInterfaceOptions const &Opts,
                                             ModuleDecl *M) {
   auto &Ctx = M->getASTContext();
-  auto ToolsVersion = swift::version::getSwiftFullVersion(
-      Ctx.LangOpts.EffectiveLanguageVersion);
+  auto ToolsVersion =
+      getSwiftInterfaceCompilerVersionForCurrentCompiler(Ctx);
   out << "// " SWIFT_INTERFACE_FORMAT_VERSION_KEY ": "
       << InterfaceFormatVersion << "\n";
   out << "// " SWIFT_COMPILER_VERSION_KEY ": "
@@ -71,6 +71,12 @@ static void printToolVersionAndFlagsComment(raw_ostream &out,
       << Opts.Flags << "\n";
 }
 
+std::string
+swift::getSwiftInterfaceCompilerVersionForCurrentCompiler(ASTContext &ctx) {
+  return swift::version::getSwiftFullVersion(
+             ctx.LangOpts.EffectiveLanguageVersion);
+}
+
 llvm::Regex swift::getSwiftInterfaceFormatVersionRegex() {
   return llvm::Regex("^// " SWIFT_INTERFACE_FORMAT_VERSION_KEY
                      ": ([0-9\\.]+)$", llvm::Regex::Newline);
@@ -81,6 +87,11 @@ llvm::Regex swift::getSwiftInterfaceModuleFlagsRegex() {
                      llvm::Regex::Newline);
 }
 
+llvm::Regex swift::getSwiftInterfaceCompilerVersionRegex() {
+  return llvm::Regex("^// " SWIFT_COMPILER_VERSION_KEY
+                     ": (.+)$", llvm::Regex::Newline);
+}
+
 /// Prints the imported modules in \p M to \p out in the form of \c import
 /// source declarations.
 static void printImports(raw_ostream &out,