Clean up "const" situation in format_args!().
This cleans up the "const" situation in the format_args!() expansion/lowering.
Rather than marking the Argument::new_display etc. functions as non-const, this marks the Arguments::new_v1 functions as non-const.
Example expansion/lowering of format_args!() in const:
```rust
// Error: cannot call non-const formatting macro in constant functions
const {
fmt::Arguments::new_v1( // Now the error is produced here.
&["Hello, ", "!\n"],
&[
fmt::Argument::new_display(&world) // The error used to be produced here.
],
)
}
```
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