…=GuillaumeGomez

[beta] rustdoc: semi-revert libsyntax doctest parsing if a macro is wrapping main

Fixes #56898

This is a patch to doctest parsing on beta (that i plan to "forward-port" to master) that reverts to the old text-based `fn main` scan if it found a macro invocation in the doctest but did not find a main function. This should solve situations like `allocator_api` which wrap their main functions in a macro invocation, but doesn't solve something like the initial version of `quicli` which used a macro to *generate* the main function. (Properly doing the latter involves running macro expansion before checking, which is a bit too involved for a beta fix.)

…-parsing, r=GuillaumeGomez

rustdoc: use text-based doctest parsing if a macro is wrapping main

This is a "forward-port" of rust-lang#57019, intended to get rust-lang#56898 on nightly, since it's now fixed on beta (and already worked on stable).

To recap:

* The libsyntax-based doctest parsing now checks to see whether there is a top-level macro invocation in the doctest while it's checking for `fn main` and an `extern crate` statement.
* If it finds a macro invocation and *didn't* find `fn main`, then it performs the older text-based scan to allow doctests like the ones in `allocator_api` to still compile.

A "proper" fix will involve changing how `make_test` works to call it later in the `run_test` function, after the initial steps of compilation have completed. I've filed [a separate issue](rust-lang#57415) for that, though.