[llvm-objdump] Add warning if --disassemble-functions specifies an unknown symbol

Summary:
Fixes Bug 41904 https://bugs.llvm.org/show_bug.cgi?id=41904

Re-land r362768 after it was reverted in r362826.

Reviewers: jhenderson, rupprecht, grimar, MaskRay

Reviewed By: jhenderson, rupprecht, MaskRay

Subscribers: dexonsmith, rupprecht, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62275

llvm-svn: 362838