Use a separate interner type for UniqueTypeId

Using symbol::Interner makes it very easy to mixup UniqueTypeId symbols
with the global interner. In fact the Debug implementation of
UniqueTypeId did exactly this.

Using a separate interner type also avoids prefilling the interner with
unused symbols and allow for optimizing the symbol interner for parallel
access without negatively affecting the single threaded module codegen.

compiler/rustc_span/src/symbol.rs

@@ -1701,8 +1701,11 @@ impl<CTX> ToStableHashKey<CTX> for Symbol {
 // The `FxHashMap`+`Vec` pair could be replaced by `FxIndexSet`, but #75278
 // found that to regress performance up to 2% in some cases. This might be
 // revisited after further improvements to `indexmap`.
+//
+// This type is private to prevent accidentally constructing more than one `Interner` on the same
+// thread, which makes it easy to mixup `Symbol`s between `Interner`s.
 #[derive(Default)]
-pub struct Interner {
+pub(crate) struct Interner {
     arena: DroplessArena,
     names: FxHashMap<&'static str, Symbol>,
     strings: Vec<&'static str>,