Represent integer literals as IntLiteral not as i32. (#4532)

When an `IntLiteral` appears as an operand of an `if` expression,
convert it to `i32` for now, so that we don't reject things like `if
cond then 1 else 2` due to having a non-constant value of type
`IntLiteral`.

For tuple indexing expressions such as `(a, b).0`, convert the index to
type `IntLiteral`, not to type `i32`. This isn't strictly necessary to
do in this PR, but avoids the need to provide an `IntLiteral` -> `i32`
implicit conversion for `no_prelude` tests using this syntax.

toolchain/diagnostics/coverage_test.cpp

@@ -29,10 +29,6 @@ constexpr DiagnosticKind UntestedDiagnosticKinds[] = {
     DiagnosticKind::ErrorStattingFile,
     DiagnosticKind::FileTooLarge,
 
-    // Int literals are currently limited to i32. Once that's fixed, this
-    // should be tested.
-    DiagnosticKind::ArrayBoundTooLarge,
-
     // These aren't feasible to test with a normal testcase, but are tested in
     // lex/tokenized_buffer_test.cpp.
     DiagnosticKind::TooManyTokens,