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/formatter.cpp

@@ -866,7 +866,7 @@ class FormatterImpl {
     FormatTrailingBlock(inst.decl_block_id);
   }
 
-  auto FormatInstRHS(IntLiteral inst) -> void {
+  auto FormatInstRHS(IntValue inst) -> void {
     out_ << " ";
     sem_ir_.ints()
         .Get(inst.int_id)