[ASTGen] Infrastructure to implment ASTGen Incrementally

* '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

include/module.modulemap

@@ -10,6 +10,12 @@ module ASTBridging {
   export *
 }
 
+module ParseBridging {
+  header "swift/Parse/ParseBridging.h"
+  requires cplusplus
+  export *
+}
+
 module SILBridging {
   header "swift/SIL/SILBridging.h"
   requires cplusplus