[llvm-objcopy] - Check dynamic relocation sections for broken references.

[llvm-objcopy] - Check dynamic relocation sections for broken references.

This is a fix for https://bugs.llvm.org/show_bug.cgi?id=41371.

Currently, it is possible to break the sh_link field of the dynamic relocation section
by removing the section it refers to. The patch fixes an issue and adds 2 test cases.

One of them shows that it does not seem possible to break the sh_info field.
I added an assert to verify this.

Differential revision: https://reviews.llvm.org/D60825

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359552 91177308-0d34-0410-b5e6-96231b3b80d8