Debug info: Add a new explicit DIDescriptor flag for the "public" access
specifier and change the default behavior to only emit the
DW_AT_accessibility(public) attribute when the isPublic() is explicitly
set.

rdar://problem/18154959

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216799 91177308-0d34-0410-b5e6-96231b3b80d8