Rename _fe_analyzer_shared's ErrorCode

[srawlins]

The different "types" of diagnostics that the analyzer (and CFE?) can report exist as classes (like LintCode, ParserErrorCode, TodoCode, StaticWarningCode, CompileTimeErrorCode, WarningCode, ScannerErrorCode, AnalysisOptionsErrorCode, AnalysisOptionsWarningCode, etc), which all have a common super-class: ErrorCode.

This name is potentially confusing, as we can say, for example "using a @visibleForTesting element outside of a test does not yield an error; it yields a warning." But warning is a subclass of error? For years this confusion would only hamper the developers of the analyzer (and CFE?). But the new analyzer plugin system uses LintCode and WarningCode, and we need a better name for the shared super-class, for function signatures, etc.

I propose DiagnosticCode. It's longer, more syllables, but it is not a name that will be used often.

We can do a gentle incremental migration, and there is no rush to delete ErrorCode:

1. Add a type alias, typedef DiagnosticCode = ErrorCode.
2. Change function signatures in API that will become public for analyzer plugins.

Then, as needed, as time permits:

3. Migrate internal usage to DiagnosticCode.
4. Deprecate ErrorCode.
5. In a major version release, without any rush, rename ErrorCode to DiagnosticCode and remove the type alias.

[johnniwinther]

@srawlins would you be willing to take on this task?

[srawlins]

Absolutely I would! 😁

[srawlins]

The next big one to rename might be AnalysisError, in the analyzer package.

This is sort of the "instances" of DiagnosticCodes. Each AnalysisError represents a reported diagnostic. In fact, AnalysisError extends a class, Diagnostic, also defined in the analyzer package. This is curious...

Diagnostic has exactly one direct subclass (AnalysisError has a few more for tests). Diagnostic 7 times in all of analyzer. Maybe someone started attempting this migration already?

My proposal for AnalysisError: scoop all of its implementation into Diagnostic:

1. Remove unused members (correction?). (Well, deprecate; it's public API.)
2. Move used members to Diagnostic.
3. Make its subclasses sublcass Diagnostic instead.
4. Deprecate the whole class.
5. Remove the whole class.