The field `IsForASTGen` only exists when `SWIFT_BUILD_SWIFT_SYNTAX` is
defined, but the usage was unprotected. The whole function can be inside
the protected block because it only seems to be invoked from inside
blocks already protected by `SWIFT_BUILD_SWIFT_SYNTAX`.
Fix for #69761
* '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
