Trust the arange accelerator tables in dSYMs

When ingesting aranges from a dSYM it makes sense to 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

llvm-svn: 374117