Fix Metal 8-bit vector type names: emit char/uchar instead of int8_t/uint8_t (#8223)

The Metal backend was generating incorrect type names for 8-bit vector
types, causing compilation failures when targeting Metal. According to
the Metal specification, 8-bit vector types should be named `charN` and
`ucharN` (e.g., `char2`, `uchar3`) rather than `int8_tN` and `uint8_tN`.

## Problem

When compiling Slang code with 8-bit vector types for Metal, the
compiler would emit:
```metal
uint8_t2 _S8 = uint8_t2(uint8_t(0U), uint8_t(16U));
int8_t3 _S9 = int8_t3(int8_t(0), int8_t(16), int8_t(48));
```

But the Metal compiler expects:
```metal
uchar2 _S8 = uchar2(uint8_t(0U), uint8_t(16U));
char3 _S9 = char3(int8_t(0), int8_t(16), int8_t(48));
```

This caused errors like:
```
error: unknown type name 'uint8_t2'; did you mean 'uint8_t'?
```

## Solution

Modified `MetalSourceEmitter::emitSimpleTypeImpl()` to emit the correct
Metal-specific type names for 8-bit types:
- `kIROp_Int8Type` now emits `char` instead of `int8_t`
- `kIROp_UInt8Type` now emits `uchar` instead of `uint8_t`

This change only affects the Metal backend and ensures that vector types
like `int8_t2`, `uint8_t3`, etc. are correctly emitted as `char2`,
`uchar3`, etc.

## Testing

- Added a new test case `tests/metal/8bit-vector-types.slang` to verify
the fix
- Re-enabled the previously disabled Metal test in
`tests/hlsl-intrinsic/countbits8.slang`
- Updated `tests/metal/byte-address-buffer.slang` to expect the correct
type names
- Verified that existing Metal tests continue to pass

Fixes #8211.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bmillsNV <163073245+bmillsNV@users.noreply.github.com>

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