Error code files (`diagnostics.rs`) reserve a valuable module name

[petrochenkov]

... that can be used for diagnostics-related code in general, rather than for error code descriptions specifically.

Suggestion: rename the files with error codes (there are about 10 of them) to error_codes.rs to free the name diagnostics for something more useful.

Additionally: rename librustc_resolve/error_reporting.rs to librustc_resolve/diagnostics.rs.