[asan] In llvm.asan.globals, allow entries to be non-GlobalVariable and skip over them

Looks like there are valid reasons why we need to allow bitcasts in llvm.asan.globals, see discussion at https://github.com/apple/swift-llvm/pull/133. Let's look through bitcasts when iterating over entries in the llvm.asan.globals list.

Differential Revision: https://reviews.llvm.org/D55794

llvm-svn: 349544