Optimize `failureBreakpoint()`.

failureBreakpoint() currently has optimizations disabled which produces some significant assembly for what's supposed to be a no-op. Enabling optimizations and marking its magic value as @exclusivity(unchecked) reduces it to a single indirect store.

This is probably not the hottest hot path in the library (since it will only be called when a test is failing) but the whole point of the function is to be a no-op, and without this change it's quite noisy and includes a whole function call into the Swift runtime (see the bl instructions below.)

arm64

Before

_$s7Testing17failureBreakpointyyF:
0000000000000660    sub sp, sp, #0x30
0000000000000664    stp x29, x30, [sp, #0x20]
0000000000000668    add x29, sp, #0x20
000000000000066c    adrp    x8, 0 ; 0x0
0000000000000670    add x0, x8, #0x0
0000000000000674    add x1, sp, #0x8
0000000000000678    mov x2, #0x21
000000000000067c    mov x3, #0x0
0000000000000680    bl  0x680
0000000000000684    mov x8, #0x1
0000000000000688    adrp    x9, 0 ; 0x0
000000000000068c    add x9, x9, #0x0
0000000000000690    str x8, [x9]
0000000000000694    add x0, sp, #0x8
0000000000000698    bl  0x698
000000000000069c    ldp x29, x30, [sp, #0x20]
00000000000006a0    add sp, sp, #0x30
00000000000006a4    ret

After

_$s7Testing17failureBreakpointyyF:
0000000000000660    adrp    x8, 0 ; 0x0
0000000000000664    str xzr, [x8]
0000000000000668    ret

Note this disassembly comes from the intermediate .o file generated for the relevant source file, so addresses aren't available yet. Neither function actually attempts to write to 0x0.

Checklist:

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

swiftlang / swift-testing