Add more tests for cfg_boolean_literals

tests/ui/cfg/both-true-false.rs

@@ -0,0 +1,14 @@
+/// Test that placing a `cfg(true)` and `cfg(false)` on the same item result in
+//. it being disabled.`
+
+#[cfg(false)]
+#[cfg(true)]
+fn foo() {}
+
+#[cfg(true)]
+#[cfg(false)]
+fn foo() {}
+
+fn main() {
+    foo();  //~ ERROR cannot find function `foo` in this scope
+}

tests/ui/cfg/both-true-false.stderr

@@ -0,0 +1,9 @@
+error[E0425]: cannot find function `foo` in this scope
+  --> $DIR/both-true-false.rs:13:5
+   |
+LL |     foo();
+   |     ^^^ not found in this scope
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0425`.

tests/ui/cfg/cmdline-false.rs

@@ -0,0 +1,9 @@
+/// Test that `--cfg false` doesn't cause `cfg(false)` to evaluate to `true`
+//@ compile-flags: --cfg false
+
+#[cfg(false)]
+fn foo() {}
+
+fn main() {
+    foo();  //~ ERROR cannot find function `foo` in this scope
+}

tests/ui/cfg/cmdline-false.stderr

@@ -0,0 +1,9 @@
+error[E0425]: cannot find function `foo` in this scope
+  --> $DIR/cmdline-false.rs:8:5
+   |
+LL |     foo();
+   |     ^^^ not found in this scope
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0425`.