[PECOFF] Use the first definition of an export if specified multiple times.

If the same symbol is specified multiple times as arguments of /export, the
first definition should be used.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197998 91177308-0d34-0410-b5e6-96231b3b80d8
Use EXPECT_EQ in unit tests.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197999 91177308-0d34-0410-b5e6-96231b3b80d8