[Bounds-Safety] Reserve slot in SanitizerHandler enum for Bounds-Safety (#91032)

Due to how `CodeGenFunction::EmitTrapCheck` is implemented
`SanitizerHandler` with numeric value 0x19 needs to be reserved because
`-fbounds-safety` generates trap instructions with that value embedded
in the trap instructions for x86_64 and arm64 just like for UBSan traps.

** x86_64 **

```
ud1l   0x19(%eax), %eax
```

** arm64 **

```
brk    #0x5519
```

To avoid upstream Clang and AppleClang diverging their ABIs for
`-fbounds-safety` the slot is being reserved in this patch.

`SanitizerHandler::BoundsSafety` currently has no uses in the code but
uses will be introduced when the CodeGen side of `-fbounds-safety`'s
implementation is upstreamed.

rdar://126884014



Co-authored-by: Dan Liew <dan@su-root.co.uk>