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

llvm-svn: 328820