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/control_flow.cpp

@@ -93,7 +93,7 @@ auto SetBlockArgResultBeforeConstantUse(Context& context,
   // Determine the constant result based on the condition value.
   SemIR::ConstantId const_id = SemIR::ConstantId::NotConstant;
   auto cond_const_id = context.constant_values().Get(cond_id);
-  if (!cond_const_id.is_template()) {
+  if (!cond_const_id.is_concrete()) {
     // Symbolic or non-constant condition means a non-constant result.
   } else if (auto literal = context.insts().TryGetAs<SemIR::BoolLiteral>(
                  context.constant_values().GetInstId(cond_const_id))) {