Disable GC and ICF when /debug is present

ICF and GC impair debugging, so MSVC disables these optimizations when
/debug is passed. They are still on by default when no PDB is produced.

This change also makes /opt:ref enable ICF, which is consistent with
MSVC: https://msdn.microsoft.com/en-us/library/bxwfs976.aspx

We should consider making /opt:icf fold readonly data in the near
future. LLD used to do this, but we disabled it because it breaks too
many programs. MSVC only does this if the user explicitly passes
/opt:icf.

Reviewers: ruiu, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39885

llvm-svn: 318071