Improve diagnostics when intra-doc-resolution fails

[jyn514]

Similar to but not the same as #74207. Right now rustdoc reports the same error for all the following cases:

/// [<invalid syntax>]
/// [path::to::nonexistent::module]
/// [f::A]
/// [S::A]
/// [S::fmt]
pub fn f() {}
#[derive(Debug)]
pub struct S;

warning: unresolved link to `path::to::nonexistent::module`
 --> /dev/stdin:2:6
  |
2 | /// [path::to::nonexistent::module]
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unresolved link
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

However we can give much more helpful errors:

1 | /// [<invalid syntax>]
  |     ^^^^^^^^^^^^^^^^^^ not valid syntax for an intra-doc link
2 | /// [path::to::nonexistent::module]
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared type or module `path` 
3 | /// [f::A]
  |     ^^^^^^ functions cannot have associated items
4 | /// [S::A]
  |     ^^^^^^ `S` does not have an associated item `A` in scope. help: try importing a trait that defines that item
5 | /// [S::fmt]
  |     ^^^^^^ `S` does not have an associated item `fmt` in scope. help: S implements `std::fmt::Debug`, try importing it.

Most of these do not require special help from rustc_resolve (that would just let rustdoc duplicate less code).