Omit "Contents of" headers when -no-leading-headers is specified.

Summary:
llvm-objdump -macho will no longer print "Contents of" headers when
disassembling section contents when -no-leading-headers is specified.
For historical reasons, this flag is independent of -no-leading-addr.

Reviewers: ab, pete, jhenderson

Reviewed By: jhenderson

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73574