Can't pass .dSYM directory to llvm-objdump -dsym= (and error message is wrong)

Summary:
Allow users to pass the path to a .dSYM directory to llvm-objdump's -dsym
flag rather than requiring users to find the DWARF DSYM Mach-O within the
bundle structure by hand.

rdar://46873333

Reviewers: pete, lhames, friss, aprantl

Reviewed By: pete, aprantl

Subscribers: MaskRay, aprantl, rupprecht, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67780

llvm-svn: 372421