[lld][macho] Stop grouping symbols by sections in mapfile.

[lld][macho] Stop grouping symbols by sections in mapfile.

As per [Bug 50689](https://bugs.llvm.org/show_bug.cgi?id=50689),

```
2. getSectionSyms() puts all the symbols into a map of section -> symbols, but this seems unnecessary. This was likely copied from the ELF port, which prints a section header before the list of symbols it contains. But the Mach-O map file doesn't print these headers.
```

This diff removes `getSectionSyms()` and keeps all symbols in a flat vector.

What does ld64's mapfile look like?
```
$ llvm-mc -filetype=obj -triple=x86_64-apple-darwin test.s -o test.o
$ llvm-mc -filetype=obj -triple=x86_64-apple-darwin foo.s -o foo.o
$ ld -map map test.o foo.o -o out -L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib -lSystem
```

```
[  0] linker synthesized
[  1] test.o
[  2] foo.o
0x100003FB7     0x00000001      __TEXT  __text
0x100003FB8     0x00000000      __TEXT  obj
0x100003FB8     0x00000048      __TEXT  __unwind_info
0x100004000     0x00000001      __DATA  __common
0x100003FB7     0x00000001      [  1] _main
0x100003FB8     0x00000000      [  2] _foo
0x100003FB8     0x00000048      [  0] compact unwind info
0x100004000     0x00000001      [  1] _number
```

Perf numbers when linking chromium framework on a 16-Core Intel Xeon W Mac Pro:
```
base           diff           difference (95% CI)
sys_time   1.406 ± 0.020  1.388 ± 0.019  [  -1.9% ..   -0.6%]
user_time  5.557 ± 0.023  5.914 ± 0.020  [  +6.2% ..   +6.6%]
wall_time  4.455 ± 0.041  4.436 ± 0.035  [  -0.8% ..   -0.0%]
samples    35             35
```

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D114735