This is consistent with std::formatv and allows formatters to support a
wider variety of use cases (like having a bare string in their formatter
if that's useful, etc).

Came up in the context of some Carbon diagnostic work here:
https://github.com/carbon-language/carbon-lang/pull/4411#discussion_r1803688859

Fix bugs found when actually trying to use formatv() automatic index
assignment in IntrinsicEmitter.cpp:
- Assign automatic index only for `ReplacementType::Format`.
- Make the check for all replacement indices being either automatic orexplicit more accurate.
  The existing check fails for formatv("{}{0}{}", 0, 1) (added as a unit test). Explicitly track if we
  have seen any explicit and any automatic index instead.

Make index in replacement field optional. It will be automatically
assigned in incremental order by formatv.

Make mixed use of automatic and explicit indices an error that will fail
validation.
Adopt uses of formatv() within FormatVariadic to use automatic index.

- Rename `Align` field in ReplacementItem/FmtAlign to `Width` to 
  accurately reflect its use.
- Change both `Width` and `Index` in ReplacementItem to 32-bit int 
  instead of size_t (as 64-bits seems excessive in this context).
- Eliminate the use of `Empty` ReplacementType, and use the
  existing std::optional<> instead to indicate that.
- Eliminate some boilerplate type code in formatv().
- Eliminate the loop in `splitLiteralAndReplacement`. The existing
  code will never loop back.
- Directly use constructor instead of std::make_pair.

Reverts llvm/llvm-project#106589
The fix for bot failures caused by the reverted commit was committed
already, so this revert is not needed.

Reverts llvm/llvm-project#105745

Some bots are broken apparently.

Change formatv() to validate that the number of arguments passed matches
number of replacement fields in the format string, and that the replacement
indices do not contain holes.

To support cases where this cannot be guaranteed, introduce a formatv()
overload that allows disabling validation with a bool flag as its first argument.

- When an unterminated open { is detected in the format string, instead
of asserting and ignoring the error, replace that string with another to
indicate the error, and remove the assert as well.
- This will make the error evident in both assert and release builds and
make observing the error more convenient (as several uses of this
function are in TableGen and it is often built in release mode even in
debug builds)

Some support code, e.g. llvm/Support/Endian.h, uses
llvm::support::detail, but the format-related code uses llvm::detail. On
VS2019, when a C++ file includes both headers, a `detail::` from
`namespace llvm { ... }` becomes ambiguous.

44253a9c breaks TensorFlow and
[JAX](https://github.com/google/jax/actions/runs/8507773013/job/23300219405)
build because of this.

Since llvm::X::detail seems like a cleaner solution and is used in other
places as well (e.g. llvm::yaml::detail), we should probably migrate all
llvm::detail usages to llvm::X::detail.