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/dump.cpp

@@ -22,7 +22,7 @@ static auto DumpNoNewline(const File& file, ConstantId const_id) -> void {
   if (const_id.is_symbolic()) {
     llvm::errs() << ": "
                  << file.constant_values().GetSymbolicConstant(const_id);
-  } else if (const_id.is_template()) {
+  } else if (const_id.is_concrete()) {
     llvm::errs() << ": "
                  << file.insts().Get(
                         file.constant_values().GetInstId(const_id));