[NFC-ish] Simplify module loader filename handling (#29295)

* Remove dead ModuleSourceInfoFilename parameters

These were never actually used; we might find a way to bring them back later.

* Introduce SerializedModuleBaseName

This is intended to replace the _n_ filename parameters that tend to get passed around in the SerializedModuleLoader classes.

* Manipulate currPath in SerializedModuleLoader less often

* Don’t pass raw paths around SerializedModuleLoader

Only pass base names.

* Regularize module file opening functions

unittests/FrontendTool/ModuleLoadingTests.cpp

@@ -113,8 +113,8 @@ protected:
     std::unique_ptr<llvm::MemoryBuffer> moduleSourceInfoBuffer;
 
     auto error =
-      loader->findModuleFilesInDirectory({moduleName, SourceLoc()}, tempDir,
-        "Library.swiftmodule", "Library.swiftdoc", "Library.swiftsourceinfo",
+      loader->findModuleFilesInDirectory({moduleName, SourceLoc()},
+        SerializedModuleBaseName(tempDir, SerializedModuleBaseName("Library")),
         /*ModuleInterfacePath*/nullptr,
         &moduleBuffer, &moduleDocBuffer, &moduleSourceInfoBuffer);
     ASSERT_FALSE(error);