doctest: Reset errors before dropping the parse session
doctest: Reset errors before dropping the parse session

The first parse is to collect whether the code contains macros, has
`main`, and uses other crates. In that pass we ignore errors as those
will be reported when the test file is actually built.

For that we need to reset errors in the `Diagnostic` otherwise when
dropping it unhandled errors will be reported as compiler bugs.

Fixes #80992

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust