Fixes #64936 : doc comments picked up from previous scanner run

Under some circumstances it was possible for doc comments to be
associated with the wrong thing, as the doc_comment compiler global
was carried over between scanner runs.

This patch adds the doc_comment (and doc_comment_len) to
zend_lex_state, so they're saved/restored when changing contexts,
and then resets them whenever we start scanning a new file/string.