Merge `$<Feature>` and `hasFeature` implementations.
- `$<Feature>` did not support upcoming language features.
- `hasFeature` did not support promoted language features and also
didn't take into account `Options` in `Features.def`.
Remove `Options` entirely, it was always one of three cases:
- `true`
- `langOpts.hasFeature`
- `hasSwiftSwiftParser`
Since `LangOptions::hasFeature` should always be used anyway, it's no
longer necessary. `hasSwiftSwiftParser` can be special cased when adding
the default promoted language features (by removing those features).
Resolves rdar://117917456.
* 'ASTGenVisitor' has a reference to a legacy C++ Parser configured for
ASTGen.
* If 'ASTGenVisitor' encounters a AST node that hasn't been migrated,
call parse(Decl|Stmt|Expr|Type) to parse the position using the legacy
parser.
* The legacy parser calls ASTGen's
'swift_ASTGen_build(Decl|Stmt|Expr|Type)' for each ASTNode "parsing"
(unless the call is not directly from the ASTGen.)
rdar://117151886
