[LTO] Ignore -plugin/-plugin-opt options.
[LTO] Ignore -plugin/-plugin-opt options.

This is required to get 'clang -flto' to work transparently
with lld. Please refer to the short comment in the code
for a more detailed explanation.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@263862 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld