Rollup merge of #78061 - wesleywiser:opt_zst_const_interning, r=oli-obk

Optimize const value interning for ZST types

Interning can skip any inhabited ZST type in general.

Fixes #68010

r? @oli-obk

src/test/ui/issues/issue-68010-large-zst-consts.rs

@@ -0,0 +1,5 @@
+// build-pass
+
+fn main() {
+    println!("{}", [(); std::usize::MAX].len());
+}