Rollup merge of #34403 - jonathandturner:move_liberror, r=alexcrichton

This PR refactors the 'errors' part of libsyntax into its own crate (librustc_errors).  This is the first part of a few refactorings to simplify error reporting and potentially support more output formats (like a standardized JSON output and possibly an --explain mode that can work with the user's code), though this PR stands on its own and doesn't assume further changes.

As part of separating out the errors crate, I have also refactored the code position portion of codemap into its own crate (libsyntax_pos).  While it's helpful to have the common code positions in a separate crate for the new errors crate, this may also enable further simplifications in the future.

diff --cc src/librustc/hir/lowering.rs
Simple merge

diff --cc src/librustc_lint/lib.rs
Simple merge

diff --cc src/librustc_resolve/build_reduced_graph.rs
Simple merge

diff --cc src/librustc_resolve/lib.rs
Simple merge

diff --cc src/librustc_save_analysis/dump_visitor.rs
Simple merge

diff --cc src/librustc_save_analysis/lib.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/ext/base.rs
Simple merge

diff --cc src/libsyntax/ext/build.rs
Simple merge

diff --cc src/libsyntax/ext/expand.rs
Simple merge

diff --cc src/libsyntax/ext/tt/macro_rules.rs
Simple merge

diff --cc src/libsyntax/fold.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge

diff --cc src/libsyntax/visit.rs
Simple merge