Move --relocatable-pch to Options.td.

It is a valid driver option (we have tests for that), so it should not be
marked as NoDriverOption.

No functionality change other than it showing in --help.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191377 91177308-0d34-0410-b5e6-96231b3b80d8