[PECOFF] Fix duplicate /export options

If two or more /export options are given for the same symbol, we should
always print a warning message and use the first one regardless of other
parameters.
Previously there was a case that the first parameter is not used.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218342 91177308-0d34-0410-b5e6-96231b3b80d8