Auto merge of #106621 - ozkanonur:enable-elided-lifetimes-for-doctests, r=Mark-Simulacrum

enable `rust_2018_idioms` lint group for doctests

With this change, `rust_2018_idioms` lint group will be enabled for compiler/libstd doctests.

Resolves #106086
Resolves #99144

Signed-off-by: ozkanonur <work@onurozkan.dev>

Author: bors