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/lower/file_context.cpp

@@ -534,7 +534,7 @@ static auto BuildTypeForInst(FileContext& context, SemIR::FloatType /*inst*/)
 static auto BuildTypeForInst(FileContext& context, SemIR::IntType inst)
     -> llvm::Type* {
   auto width =
-      context.sem_ir().insts().TryGetAs<SemIR::IntLiteral>(inst.bit_width_id);
+      context.sem_ir().insts().TryGetAs<SemIR::IntValue>(inst.bit_width_id);
   CARBON_CHECK(width, "Can't lower int type with symbolic width");
   return llvm::IntegerType::get(
       context.llvm_context(),