[MachO] Reduce size of Symbol and Defined

We can lay out Symbol more optimally to reduce its size from 56 bytes to
48 bytes by eliminating unnecessary padding, and we can lay out Defined
such that its bitfield members are placed in the tail padding of Symbol
(on ABIs which support this), to reduce it from 96 bytes to 80 bytes (8
bytes from the Symbol reduction, and 8 bytes from the tail padding
reuse).

This is perf-neutral for an internal app (results from two different
machines):

```
           smol-syms       baseline        difference (95% CI)
sys_time   7.430 ± 0.202   7.440 ± 0.193   [  -2.6% ..   +2.9%]
user_time  21.443 ± 0.513  21.206 ± 0.396  [  -3.3% ..   +1.1%]
wall_time  20.453 ± 0.534  20.222 ± 0.488  [  -3.7% ..   +1.5%]
samples    9               8

           smol-syms       baseline        difference (95% CI)
sys_time   3.011 ± 0.050   3.040 ± 0.052   [  -0.4% ..   +2.3%]
user_time  10.416 ± 0.075  10.496 ± 0.091  [  +0.1% ..   +1.4%]
wall_time  12.229 ± 0.144  12.354 ± 0.192  [  -0.1% ..   +2.1%]
samples    14              13
```

However, on the first machine, it reduces maximum resident set size by
65.9 MB (0.8%, from 7.92 GB to 7.85 GB). On the second machine, it
reduces it by 92 MB (1.4%, from 6.40 GB to 6.31 GB).

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D113813