[lld-macho] Have dead-stripping work with literal sections

Literal sections are not atomically live or dead. Rather,
liveness is tracked for each individual literal they contain. CStrings
have their liveness tracked via a `live` bit in StringPiece, and
fixed-width literals have theirs tracked via a BitVector.

The live-marking code now needs to track the offset within each section
that is to be marked live, in order to identify the literal at that
particular offset.

Numbers for linking chromium_framework on my 3.2 GHz 16-Core Intel Xeon W
with both `-dead_strip` and `--deduplicate-literals`, with and without this diff
applied:

```
    N           Min           Max        Median           Avg        Stddev
x  20          4.32          4.44         4.375         4.372    0.03105174
+  20           4.3          4.39          4.36        4.3595   0.023277502
No difference proven at 95.0% confidence
```
This gives us size savings of about 0.4%.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D103979

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