rustdoc: Properly inline const items

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821
Merge pull request #19774 from tomjakubowski/rustdoc-consts-statics

rustdoc: Properly inline const items

Reviewed-by: alexcrichton