Fix <rdar://problem/5924576> clang -fsyntax-only generates "redefinition" errors when parsing AppKit that gcc does not.

Teach Sema::MergeVarDecl() about __private_extern__.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51005 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed