[gn build] Add build files for bugpoint-passes and LLVMHello plugins

These two plugins are loaded into a host process that contains all LLVM
symbols, so they don't link against anything. This required minor readjustments
to the tablegen() setup of IR.

Needed for check-llvm.

Differential Revision: https://reviews.llvm.org/D56204

llvm-svn: 350234