[lld-macho] Implement ICF

ICF = Identical C(ode|OMDAT) Folding

This is the LLD ELF/COFF algorithm, adapted for MachO. So far, only `-icf all` is supported. In order to support `-icf safe`, we will need to port address-significance tables (`.addrsig` directives) to MachO, which will come in later diffs.

`check-{llvm,clang,lld}` have 0 regressions for `lld -icf all` vs. baseline ld64.

We only run ICF on `__TEXT,__text` for reasons explained in the block comment in `ConcatOutputSection.cpp`.

Here is the perf impact for linking `chromium_framekwork` on a Mac Pro (16-core Xeon W) for the non-ICF case vs. pre-ICF:
```
    N           Min           Max        Median           Avg        Stddev
x  20          4.27          4.44          4.34         4.349   0.043029977
+  20          4.37          4.46         4.405        4.4115   0.025188761
Difference at 95.0% confidence
        0.0625 +/- 0.0225658
        1.43711% +/- 0.518873%
        (Student's t, pooled s = 0.0352566)
```

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D103292