Allow one or both conditional operands to be unknown

In #52 we relaxed the rule that any unknown value would immediately cause
an early exit, which in turn requires the type checker for each node to
specifically deal with unknown values.

However, we missed dealing with the check that both types match in a
conditional, causing a conditional to fail if either side of it is
unknown.

This causes errors in Terraform, as described in
hashicorp/terraform#14399.

check_types.go

@@ -414,7 +414,7 @@ func (tc *typeCheckConditional) TypeCheck(v *TypeCheck) (ast.Node, error) {
 	}
 
 	// The types of the true and false expression must match
-	if trueType != falseType {
+	if trueType != falseType && trueType != ast.TypeUnknown && falseType != ast.TypeUnknown {
 
 		// Since passing around stringified versions of other types is
 		// common, we pragmatically allow the false expression to dictate
@@ -460,7 +460,13 @@ func (tc *typeCheckConditional) TypeCheck(v *TypeCheck) (ast.Node, error) {
 	}
 
 	// Result type (guaranteed to also match falseType due to the above)
-	v.StackPush(trueType)
+	if trueType == ast.TypeUnknown {
+		// falseType may also be unknown, but that's okay because two
+		// unknowns means our result is unknown anyway.
+		v.StackPush(falseType)
+	} else {
+		v.StackPush(trueType)
+	}
 
 	return tc.n, nil
 }