Rename "generic instance" to "specific" throughout the toolchain. (#4165)

As discussed in toolchain meeting, we want to avoid overloading the
meaning of "instance", and "specific" was the best name we found. It's a
little unorthodox and inventive, but hopefully over time will become as
unsurprising as the term "generic" is.

toolchain/check/handle_function.cpp

@@ -335,7 +335,7 @@ static auto HandleFunctionDefinitionAfterSignature(
 
   // Check the return type is complete.
   CheckFunctionReturnType(context, function.return_storage_id, function,
-                          SemIR::GenericInstanceId::Invalid);
+                          SemIR::SpecificId::Invalid);
 
   // Check the parameter types are complete.
   for (auto param_id : llvm::concat<const SemIR::InstId>(