[Demangler] Tidy up a bit, and add line numbers to ManglingErrors.

Because DEMANGLER_ASSERT() might cause the remanglers to return a ManglingError
with the code ManglingError::AssertionFailed, it's useful to have a line number
in the ManglingError as well as the other information.  This is also potentially
helpful for other cases where the code is used multiple times in the remanglers.

rdar://79725187

tools/swift-ide-test/swift-ide-test.cpp

@@ -2398,8 +2398,9 @@ static int doPrintLocalTypes(const CompilerInvocation &InitInvok,
       auto mangling = Demangle::mangleNode(typeNode);
       if (!mangling.isSuccess()) {
         llvm::errs() << "Couldn't remangle type (failed at Node "
-                     << mangling.errorNode() << " with error "
-                     << mangling.errorCode() << ")\n";
+                     << mangling.error().node << " with error "
+                     << mangling.error().code << ":" << mangling.error().line
+                     << ")\n";
         return EXIT_FAILURE;
       }
       auto remangled = mangling.result();