[lldb] Trust the arange accelerator tables in dSYMs

When ingesting aranges from a dSYM, always trust the contents of the
accelerator table since it always comes from dsymutil.

According to Instruments, skipping the decoding of all CU DIEs to get at
the DW_AT_ranges attribute removes ~3.5 seconds from setting a
breakpoint by file/line when debugging clang with a dSYM. Interestingly
on the wall clock the speedup is less noticeable, but still present.

rdar://problem/56057688

Differential Revision: https://reviews.llvm.org/D68655