[ModuleInterface] Support aliasing a module loaded from a swiftinterface

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1027,9 +1027,10 @@ class ModuleInterfaceLoaderImpl {
       }
       // Set up a builder if we need to build the module. It'll also set up
       // the genericSubInvocation we'll need to use to compute the cache paths.
+      Identifier realName = ctx.getRealModuleName(ctx.getIdentifier(moduleName));
       ImplicitModuleInterfaceBuilder builder(
         ctx.SourceMgr, diagsToUse,
-        astDelegate, interfacePath, moduleName, cacheDir,
+        astDelegate, interfacePath, realName.str(), cacheDir,
         prebuiltCacheDir, backupInterfaceDir, StringRef(),
         Opts.disableInterfaceLock,
         ctx.IgnoreAdjacentModules, diagnosticLoc,

test/ModuleInterface/module-alias-renaming-swiftinterface.swift

@@ -0,0 +1,32 @@
+/// Check that we can alias a module imported from a swiftinterface
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module -module-name Lib \
+// RUN:     -swift-version 5 -enable-library-evolution \
+// RUN:     -o %t/Lib.swiftmodule \
+// RUN:     -emit-module-interface-path %t/Lib.swiftinterface \
+// RUN:     %t/Lib.swift
+
+/// We can alias an imported module built from a swiftmodule
+// RUN: %target-swift-frontend -typecheck -module-name Client \
+// RUN:     -swift-version 5 \
+// RUN:     -module-alias AliasedLib=Lib \
+// RUN:     %t/Client.swift -I%t
+
+/// We can alias an imported module built from a swiftinterface
+// RUN: rm %t/Lib.swiftmodule
+// RUN: %target-swift-frontend -typecheck -module-name Client \
+// RUN:     -swift-version 5 \
+// RUN:     -module-alias AliasedLib=Lib \
+// RUN:     %t/Client.swift -I%t
+
+//--- Lib.swift
+public func foo() {}
+
+//--- Client.swift
+import AliasedLib
+
+func main() {
+  AliasedLib.foo()
+}