file_name -> filename (#3791)

I wanted to choose one or the other. I think some code has been using
each from early on. We're predominately using `filename`, so
consolidating on that. This conveniently matches the [Google dev doc
style guide](https://developers.google.com/style/word-list#filename)
(which we use for docs) which says "filename: Not file name".

In toolchain:

```
╚╡git grep file_name . | wc -l
35
╚╡git grep filename . | wc -l
489
```

toolchain/source/source_buffer.cpp

@@ -13,7 +13,7 @@ namespace Carbon {
 namespace {
 struct FilenameTranslator : DiagnosticLocationTranslator<llvm::StringRef> {
   auto GetLocation(llvm::StringRef filename) -> DiagnosticLocation override {
-    return {.file_name = filename};
+    return {.filename = filename};
   }
 };
 }  // namespace