Report cannot load submodule if only able to load top module

For loaders that don't support loading submodules, canImportModule should report false instead of checking if can import top module.

Author: ApolloZhu

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1832,6 +1832,8 @@ bool ExplicitSwiftModuleLoader::canImportModule(ImportPath::Module path,
                                                 llvm::VersionTuple version,
                                                 bool underlyingVersion) {
   // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   ImportPath::Element mID = path.front();
   // Look up the module with the real name (physical name on disk);
   // in case `-module-alias` is used, the name appearing in source files

lib/Sema/SourceLoader.cpp

@@ -73,7 +73,7 @@ bool SourceLoader::canImportModule(ImportPath::Module path,
                                    llvm::VersionTuple version,
                                    bool underlyingVersion) {
   // FIXME: Swift submodules?
-  if (path.size() > 1)
+  if (path.hasSubmodule())
     return false;
 
   auto ID = path[0];

lib/Serialization/SerializedModuleLoader.cpp

@@ -1126,6 +1126,9 @@ swift::extractUserModuleVersionFromInterface(StringRef moduleInterfacePath) {
 bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
                                                  llvm::VersionTuple version,
                                                  bool underlyingVersion) {
+  // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   // If underlying version is specified, this should be handled by Clang importer.
   if (!version.empty() && underlyingVersion)
     return false;
@@ -1146,7 +1149,6 @@ bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
     unusedModuleDocBuffer = &moduleDocBuffer;
   }
 
-  // FIXME: Swift submodules?
   auto mID = path[0];
   auto found = findModule(mID, unusedModuleInterfacePath, unusedModuleBuffer,
                           unusedModuleDocBuffer, unusedModuleSourceInfoBuffer,
@@ -1185,10 +1187,12 @@ bool SerializedModuleLoaderBase::canImportModule(ImportPath::Module path,
 bool MemoryBufferSerializedModuleLoader::canImportModule(
     ImportPath::Module path, llvm::VersionTuple version,
     bool underlyingVersion) {
+  // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   // If underlying version is specified, this should be handled by Clang importer.
   if (!version.empty() && underlyingVersion)
     return false;
-  // FIXME: Swift submodules?
   auto mID = path[0];
   auto mIt = MemoryBuffers.find(mID.Item.str());
   if (mIt == MemoryBuffers.end())