[Serialization] Use the module interface as the name of the file

...rather than the buffer, for a compiled module that came from a
module interface.

This was already happening at a higher level
(ModuleDecl::getModuleFilename) so pushing it down to the low-level
ModuleFile::getModuleFilename doesn't really change things much. The
important fix that goes with this is that SerializedASTFile no longer
leaks this name by storing it outside of ModuleFile.

https://bugs.swift.org/browse/SR-11365

lib/Serialization/ModuleFile.cpp

@@ -1352,8 +1352,7 @@ ModuleFile::ModuleFile(
           break;
         }
         case input_block::PARSEABLE_INTERFACE_PATH: {
-          if (extInfo)
-            extInfo->setParseableInterface(blobData);
+          ModuleInterfacePath = blobData;
           break;
         }
         default: