Ignore apparent buffer overruns on external or weak globals. This is a major

Ignore apparent buffer overruns on external or weak globals.  This is a major
source of false positives due to globals being declared in a header with some
kind of incomplete (small) type, but the actual definition being bigger.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164912 91177308-0d34-0410-b5e6-96231b3b80d8