[ELF] Add --print-icf-sections flag

Currently ICF information is output through stderr if the "--verbose"
flag is used. This differs to Gold for example, which uses an explicit
flag to output this to stdout. This commit adds the
"--print-icf-sections" and "--no-print-icf-sections" flags and changes
the output message format for clarity and consistency with
"--print-gc-sections". These messages are still output to stderr if
using the verbose flag. However to avoid intermingled message output to
console, this will not occur when the "--print-icf-sections" flag is
used.

Existing tests have been modified to expect the new message format from
stderr.

Patch by Owen Reynolds.

Differential Revision: https://reviews.llvm.org/D42375

Reviewers: ruiu, rafael

Reviewed by: 

llvm-svn: 323976