Don't reject attribute used in an "extern const" variable definition.

Before this patch we would warn and drop the attribute in
extern const char test3[] __attribute__((used)) = "";

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188588 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed