[COFF] make /incremental control overwriting unchanged import libraries

Summary:
r323164 made lld-link not overwrite import libraries when their
contents haven't changed. MSVC's link.exe does this only when
performing incremental linking. This change makes lld-link's import
library overwriting similarly dependent on whether or not incremental
linking is being performed. This is controlled by the /incremental or
/incremental:no options. In addition, /opt:icf, /opt:ref, and /order
turn off /incremental and issue a warning if /incremental was
specified on the command line.

Reviewers: rnk, ruiu, zturner

Reviewed By: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42716

llvm-svn: 323930