[PECOFF] Warn only when /export options are not compatible.

Currently LLD always print a warning message if the same symbol is specified
more than once for /export option. It's a bit annoying because specifying the
same symbol with compatible options is actually safe and considered as a
normal use case. This patch makes LLD to warn only when incompatible export
options are given.

llvm-svn: 198104
2 files changed