Set IgnoreLLVMUsed to false in CallGraph::addToCallGraph()

clang++ uses llvm.compiler.used in certain cases to preserve
symbol which is fully inlined. D96087 has resulted in undefined
symbols in such cases. Set it to false by default to preserve
old behavior but keep the option for specific uses where we
want to ignore these (e.g. to detect a potential indirect call
to a function).

Differential Revision: https://reviews.llvm.org/D99897