[lld-macho] Check if DWARF offset is too large for compact unwind

For functions that use DWARF encodings, their compact unwind entry will
contain a hint about the offset of their DWARF entry from the start of
the `__eh_frame` section. The encoding only has 3 bytes to encode this
hint.

Previously, I neglected to check for overflow (and didn't realize that
the value was merely a hint without needing to be exact.) So for large
`__eh_frame` sections, the hint would overflow and cause the compact
unwind MODE flag to be corrupted, leading to uncaught exceptions at
runtime.

This diff fixes things by encoding zero as the hint for offsets that are
too large. The unwinder will start a linear search at the hint location
for the matching CFI record. The only requirement is that the hint
points to a valid CFI record start, and the start of the section is
always the start of a CFI record (in well-formed programs).

I'm not adding a test for this because generating the test inputs takes
a bit too much time. However, I have been testing locally with this lit
file, which takes about 15s to run on my machine:

```
# RUN: rm -rf %t; mkdir %t
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos11.0 %s -o %t/test.o
# RUN: %lld -dylib -lSystem %t/test.o -o %t/test

.subsections_via_symbols
.text
.p2align 2

_f:
  .cfi_startproc
.rept 0x7fffff
  .cfi_escape 0x2e, 0x10
.endr
  ret
  .cfi_endproc

_g:
  .cfi_startproc
  .cfi_escape 0x2e, 0x10
  ret
  .cfi_endproc
```

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D147505