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/check/import_ref.cpp

@@ -158,7 +158,7 @@ static auto GetInstWithConstantValue(const SemIR::File& file,
     return SemIR::InstId::None;
   }
 
-  // For template constants, the corresponding instruction has the desired
+  // For concrete constants, the corresponding instruction has the desired
   // constant value.
   if (!const_id.is_symbolic()) {
     return file.constant_values().GetInstId(const_id);