Add PPX for constant expressions
Add PPX for constant expressions

Summary: Add a new entry point to the parser from the Expression node.

Reviewed By: pakoito

Differential Revision: D13901245

fbshipit-source-id: 79fadfc329606da2018011e0e27b3e5e99377ddc

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow