De-virtualize GlobalValue

The erase/remove from parent methods now use a switch table to remove
themselves from their appropriate parent ilist.

The copyAttributesFrom method is now completely non-virtual, since we
only ever copy attributes from a global of the appropriate type.

Pre-requisite to de-virtualizing Value to save a vptr
(https://reviews.llvm.org/D31261).

NFC

llvm-svn: 302823