PR7927, PR16247: Reimplement handling of matching extern "C" declarations

PR7927, PR16247: Reimplement handling of matching extern "C" declarations
across scopes.

When we declare an extern "C" name that is not a redeclaration of an entity in
the same scope, check whether it redeclares some extern "C" entity from another
scope, and if not, check whether it conflicts with a (non-extern-"C") entity in
the translation unit.

When we declare a name in the translation unit that is not a redeclaration,
check whether it conflicts with any extern "C" entities (possibly from other
scopes).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185229 91177308-0d34-0410-b5e6-96231b3b80d8