Parse doctests in needless_doctest_main

[ebroto]

This switches from text-based search to running the parser to avoid false positives. Inspired by how rustdoc handles this and by #4729.

cc @llogiq

changelog: Fix multiple false positives in [needless_doctest_main].

Fixes #5879
Fixes #4906
Fixes #5103
Fixes #4698

[rust-highfive]

r? @Manishearth

(rust_highfive has picked a reviewer for you, use r? to override)

[Manishearth]

Code looks fine to me, is there a perf impact? cc @flip1995 to sanity check the approach here

[flip1995]

Perf aspect is, that Clippy now parses items in doc code. But I don't think that this has much of an impact, since there shouldn't be that many Items. If there are no items, the loop statement will directly return. This change only executes code other than return false if it finds a function, that could be main.

-> I think perf wise, this should be fine. 👍

[flip1995]

@bors r=Manishearth,flip1995

[bors]

📌 Commit 1a140dc has been approved by Manishearth,flip1995

[bors]

⌛ Testing commit 1a140dc with merge f8db258...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth,flip1995
Pushing f8db258 to master...