Add PhonyOutput to BuildParams

This simplifies creating a phony rule with commands.

Bug: b/396145326
Test: manual, TH
Change-Id: I722628c6b5af09fcab0fcb8a7f74e8877be11090

ninja_defs.go

diff --git a/ninja_defs.go b/ninja_defs.go
index b68ecee..4d29d0c 100644
--- a/ninja_defs.go
+++ b/ninja_defs.go
@@ -90,6 +90,7 @@
 	Validations     []string          // The list of validations to run when this rule runs.
 	Args            map[string]string // The variable/value pairs to set.
 	Default         bool              // Output a ninja default statement
+	PhonyOutput     bool              // This is a phony_output
 }
 
 // A poolDef describes a pool definition.  It does not include the name of the
@@ -394,6 +395,10 @@
 		}
 	}
 
+	if params.PhonyOutput {
+		setVariable("phony_output", simpleNinjaString("true"))
+	}
+
 	return b, nil
 }