Use similar scheme as DeclAttribute.
* Create `BridgedTypeAttribute.createSimple()` and
`BridgedTypeAttributes.add()`, instead of
`BridgedTypeAttributes.addSimple()`
* Create `DeclAttributes::createSimple()` to align with `TypeAttribute`
Generate all entries, and ensure we introduce a
PatternBindingInitializer context for non-local
cases. Also use this opportunity to cleanup
`PatternBindingDecl::create`.
* '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
For nullable nodes, introduce both a non-null and
nullable variant of the bridging wrapper. This
allows us to annotate the necessary parameters
as nullable, but keep the returns of the bridged
`createParsed` methods non-null.
