[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/AST/Module.cpp

@@ -1249,10 +1249,6 @@ StringRef ModuleDecl::getModuleFilename() const {
   // per-file names. Modules can consist of more than one file.
   StringRef Result;
   for (auto F : getFiles()) {
-    Result = F->getParseableInterface();
-    if (!Result.empty())
-      return Result;
-
     if (auto SF = dyn_cast<SourceFile>(F)) {
       if (!Result.empty())
         return StringRef();