[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/SerializedModuleLoader.cpp

@@ -548,7 +548,6 @@ FileUnit *SerializedModuleLoaderBase::loadAST(
     // We've loaded the file. Now try to bring it into the AST.
     auto fileUnit = new (Ctx) SerializedASTFile(M, *loadedModuleFile,
                                                 extendedInfo.isSIB());
-    fileUnit->setParseableInterface(extendedInfo.getParseableInterface());
     M.addFile(*fileUnit);
     if (extendedInfo.isTestable())
       M.setTestingEnabled();