[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

include/swift/AST/Module.h

@@ -872,10 +872,6 @@ public:
     return getParentModule()->getName().str();
   }
 
-  /// If this is a module imported from a parseable interface, return the path
-  /// to the interface file, otherwise an empty StringRef.
-  virtual StringRef getParseableInterface() const { return {}; }
-
   /// Traverse the decls within this file.
   ///
   /// \returns true if traversal was aborted, false if it completed