Clarify why we need int constants for expression types.

Change-Id: I053fc5d32c118dbdb204dc8193337f981aff796e
Include necessary Makefile.INCLUDE from rules/Makefile.

Change-Id: I077d018dbe4093cd40ddf38d66a996df222bf5e4
Rename remaining all-caps constants in AST layer.

Change-Id: Ibe97e30981969056ffcdb89e63c1468ea1ffa140