Devirtualize File::kind.

This is the standard way of implementing LLVM RTTI.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@177555 91177308-0d34-0410-b5e6-96231b3b80d8
Devirtualize Reference::kind.

Improves performance.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@177556 91177308-0d34-0410-b5e6-96231b3b80d8
memcpy instead of copy_n. Faster than copy_n on MSVC :(.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@177557 91177308-0d34-0410-b5e6-96231b3b80d8