Print out "Alias for -foo" instead of repeating the same help message for -foo.

Since aliases don't actually need name, I removed it from Options.td
to keep the definitions concise.

Before:

  -(                      Ignored for compatibility with GNU unless you pass --warn-backrefs
  -)                      Ignored for compatibility with GNU unless you pass --warn-backrefs
  --allow-multiple-definition Allow multiple definitions
  --apply-dynamic-relocs  Apply dynamic relocations to place
  --as-needed             Only set DT_NEEDED for shared libraries if used
  --auxiliary=<value>     Set DT_AUXILIARY field to the specified name
  --Bdynamic              Link against shared libraries
  --Bshareable            Build a shared object
  ...

After:

  -(                      Alias for --start-group
  -)                      Alias for --end-group
  --allow-multiple-definition Allow multiple definitions
  --apply-dynamic-relocs  Apply dynamic relocations to place
  --as-needed             Only set DT_NEEDED for shared libraries if used
  --auxiliary=<value>     Set DT_AUXILIARY field to the specified name
  --Bdynamic              Link against shared libraries (default)
  --Bshareable            Alias for --shared
  ...

Differential Revision: https://reviews.llvm.org/D47588

llvm-svn: 333694