[PM] Sink the "Expression" type for GVN into the class as a private

member type.

Because of how this type is used by the ValueTable, it cannot actually
have hidden visibility. GCC actually nicely warns about this but Clang
just silently ... I don't even know. =/ We should do a better job either
way though.

This should resolve a bunch of the GCC warnings about visibility that
the port of GVN triggered and make the visibility story a bit more
correct.

llvm-svn: 263250