mk: Ensure docs have relative links to each other

When generating documentation, rustdoc has the ability to generate relative
links within the current distribution of crates to one another. To do this, it
must recognize when a crate's documentation is in the same output directory. The
current threshold for "local documentation for crate X being available" is
whether the directory "doc/X" exists.

This change modifies the build system to have new dependencies for each
directory of upstream crates for a rustdoc invocation. This will ensure that
when building documentation that all the crates in the standard distribution are
guaranteed to have relative links to one another.

This change is prompted by guaranteeing that offline docs always work with one
another. Before this change, races could mean that some docs were built before
others, and hence may have http links when relative links would suffice.

Closes #14747
debuginfo: Generate cross-crate unique type identifiers for debuginfo types.

With this change, rustc creates a unique type identifier for types in debuginfo. These type identifiers are used by LLVM to correctly handle link-time-optimization scenarios but also help rustc with dealing with inlining from other crates. For more information, see the documentation block at the top of librustc/middle/trans/debuginfo.rs.

Fixes #13681.