Support float-like tuple indices in offset_of!()
Supports invocations like `offset_of!((((), ()), ()), 0.0)`. This `0.0` gets tokenized as float literal, so it has to be broken up again.
The code that did the breaking up was returning a finished `Expr`, while we need a `Ident`, so this PR splits up the `parse_expr_tuple_field_access_float` function into:
* a function that breaks up the float literal (similar to `TokenKind::break_two_token_op`, but we do access the parser during this splitting operation, so we keep it as an inherent function on the parser)
* and a function that constructs an `Expr` from it
The former we can then re-use in `offset_of` parsing. The edge cases especially involving whitespaces are tricky so this adds a bunch of new tests as well.
fixes#112204
43062c43d2