For llvm-nm and Mach-O files that are fully stripped, special case a redacted LC_MAIN

As a further refinement on:

r328274 - For llvm-nm and Mach-O files also use function starts info in some cases when printing symbols

we want to special case a redacted LC_MAIN so it is easier to find.

rdar://38978929


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328820 91177308-0d34-0410-b5e6-96231b3b80d8