Add an unreachable marker function.

[grynspan]

This PR adds a wrapper around __builtin_unreachable() (Builtin.unreachable() when building the Swift standard library) that we can use in place of fatalError(). The benefit is that the generated code size for unreachable paths is significantly reduced. For example, given the following function compiled with -O:

@available(*, unavailable) func f() {
  fatalError("Unreachable")
}

The compiler currently produces:

sub    sp, sp, #0x20
stp    x29, x30, [sp, #0x10]
add    x29, sp, #0x10
mov    w8, #0x1                  ; =1
str    w8, [sp, #0x8]
mov    w8, #0xc                  ; =12
str    x8, [sp]
adrp   x0, 0
add    x0, x0, #0x6a8            ; "Fatal error"
adrp   x5, 0
add    x5, x5, #0x690            ; "UnreachableTest/S.swift"
mov    x3, #0x6e55               ; =28245
movk   x3, #0x6572, lsl #16
movk   x3, #0x6361, lsl #32
movk   x3, #0x6168, lsl #48
mov    x4, #0x6c62               ; =27746
movk   x4, #0x65, lsl #16
movk   x4, #0xeb00, lsl #48
mov    w1, #0xb                  ; =11
mov    w2, #0x2                  ; =2
mov    w6, #0x17                 ; =23
mov    w7, #0x2                  ; =2
bl     0x100000680               ; symbol stub for: Swift._assertionFailure(_: Swift.StaticString, _: Swift.String, file: Swift.StaticString, line: Swift.UInt, flags: Swift.UInt32) -> Swift.Never
brk    #0x1

But with this change:

@available(*, unavailable) func f() {
  swt_unreachable()
}

It instead compiles to simply:

brk    #0x1

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[briancroom]

Cool, makes sense!