rollup merge of #21881: richo/lint-no-mangle-const

This renames the PrivateNoMangleFns lint to allow both to happen in a
single pass, since they do roughly the same work.

Closes #21856

Open questions:

[ ]: Do the tests actually pass (I'm running make check and running out the door now)
[ ]: Is the name of this lint ok. it seems to mostly be fine with [convention](cc53afbe5d/text/0344-conventions-galore.md (lints))
[ ]: I'm not super thrilled about the warning text

r? @kmcallister (Shamelessly nominating because you were looking at my other ticket)

src/test/compile-fail/lint-unexported-no-mangle.rs

@@ -8,17 +8,32 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags:-F private_no_mangle_fns
+// compile-flags:-F private_no_mangle_fns -F no_mangle_const_items -F private_no_mangle_statics
 
 // FIXME(#19495) no_mangle'ing main ICE's.
 #[no_mangle]
 fn foo() { //~ ERROR function foo is marked #[no_mangle], but not exported
 }
 
+#[allow(dead_code)]
+#[no_mangle]
+const FOO: u64 = 1; //~ ERROR const items should never be #[no_mangle]
+
+#[no_mangle]
+pub const PUB_FOO: u64 = 1; //~ ERROR const items should never be #[no_mangle]
+
 #[no_mangle]
 pub fn bar()  {
 }
 
+#[no_mangle]
+pub static BAR: u64 = 1;
+
+#[allow(dead_code)]
+#[no_mangle]
+static PRIVATE_BAR: u64 = 1; //~ ERROR static PRIVATE_BAR is marked #[no_mangle], but not exported
+
+
 fn main() {
     foo();
     bar();