LTO: Export local context symbols

Export symbols in libLTO.dylib for the local context-related functions
added in r221733 (`LTO_API_VERSION=11`)... and add the missing
definition for `lto_codegen_create_in_local_context()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224567 91177308-0d34-0410-b5e6-96231b3b80d8