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_macros/src/symbols.rs

@@ -215,7 +215,7 @@ fn symbols_with_errors(input: TokenStream) -> (TokenStream, Vec<syn::Error>) {
         }
 
         impl Interner {
-            pub fn fresh() -> Self {
+            pub(crate) fn fresh() -> Self {
                 Interner::prefill(&[
                     #prefill_stream
                 ])