Rename template constant -> concrete constant. (#4939)

This implements a direction decided in a
[recent
discussion](https://docs.google.com/document/d/1Iut5f2TQBrtBNIduF4vJYOKfw7MbS8xH_J01_Q4e6Rk/edit?resourcekey=0-mc_vh5UzrzXfU4kO-3tOjA&tab=t.0#heading=h.mas1g68xx9ct)
to switch away from "template constant" when naming a constant that
doesn't depend on any generic parameters, because that creates confusion
with template-dependent constant values that depend on a template
parameter.

toolchain/sem_ir/ids.cpp

@@ -24,11 +24,11 @@ auto ConstantId::Print(llvm::raw_ostream& out, bool disambiguate) const
     IdBase::Print(out);
     return;
   }
-  if (is_template()) {
+  if (is_concrete()) {
     if (disambiguate) {
-      out << "template_constant(";
+      out << "concrete_constant(";
     }
-    out << template_inst_id();
+    out << concrete_inst_id();
     if (disambiguate) {
       out << ")";
     }