AsmParser: restore LLVM IR compatibility for linker_private{,_weak}

This restores the linker_private and linker_private_weak lexemes to permit
translation of the deprecated lexmes.  The behaviour is identical to the bitcode
handling: linker_private and linker_private_weak are handled as if private had
been specified.  This enables compatibility with IR generated by LLVM 3.4.

Reported on IRC by ki9a!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205675 91177308-0d34-0410-b5e6-96231b3b80d8