Convert values to strings when assigning to a string variable

Currently, a string variable can easily be reassigned to
a different type. Make it so that the value it's being
reassigned to is converted to a string first.

Bug: 224601891
Test: go test
Change-Id: I82252cf9e106b5a3677458cf1df2e9d1dfefe0f6

mk2rbc/mk2rbc.go

@@ -566,6 +566,12 @@ func (ctx *parseContext) handleAssignment(a *mkparser.Assignment) []starlarkNode
 		}
 	}
 
+	if asgn.lhs.valueType() == starlarkTypeString &&
+		asgn.value.typ() != starlarkTypeUnknown &&
+		asgn.value.typ() != starlarkTypeString {
+		asgn.value = &toStringExpr{expr: asgn.value}
+	}
+
 	asgn.previous = ctx.lastAssignment(lhs)
 	ctx.setLastAssignment(lhs, asgn)
 	switch a.Type {