Rename IntLiteral to IntValue. (#4475)

This instruction represents integer values, whether they come from
literals or calculations, so it the old name is inaccurate. I also plan
to rename `BigInt` to `IntLiteral` based on recent discussion and this
change aims to avoid confusion stemming from the same name being used
for two different things.

I'm not renaming `FloatLiteral` because recent discussion suggests we
may want distinct `FloatLiteral` versus `FloatValue` representations in
SemIR.

toolchain/sem_ir/file.cpp

@@ -267,7 +267,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case InterfaceType::Kind:
       case InterfaceWitness::Kind:
       case InterfaceWitnessAccess::Kind:
-      case IntLiteral::Kind:
+      case IntValue::Kind:
       case IntType::Kind:
       case PointerType::Kind:
       case SpecificFunction::Kind: