[import_granularity] Allow formatting of use behind #[cfg*]

[dingxiangfei2009]

Fix #6666

Related to #4991

This patch fixes the issue that import_granularity does not apply import_granuarity to uses behind attributes known to the compiler that the out coding style applies transparently. Attributes like #[cfg], #[cfg_attr], #[warn], #[deny], #[allow].

We achieve this by temporarily swap out the default use tree and merge sub-items in a temporary use tree. This tree is then later merged back into the default use tree, with the original attributes grandfathered.

[matthewhughes934]

I think it would be worthwhile adding some extra tests:

• That attributes we don't want to merge are left alone, e.g. that both of the following are left alone with granularity=crate

  /// some doc comment
  use a::{b::c, d::e, d::f};
  
  #[my_atttribute]
  use a::{b::c, d::e, d::f};

• That the change works as expected across all relevant values of imports_granularity