Add LLVM_ATTRIBUTE_USED to fix problems which could be exposed by aggressive...

Add LLVM_ATTRIBUTE_USED to fix problems which could be exposed by aggressive global pointer variable removal

Note to BuryPointer.cpp:GraveYard. 'unused' cannot prevent (1) dead store
elimination and (2) removal of the global pointer variable (D69428) but 'used' can.

Discovered when comparing link maps between HEAD+D69428 and HEAD.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D101217