Implement non-internal linkage for lambda closure types that need a

Implement non-internal linkage for lambda closure types that need a
stable mangling, since these lambdas can end up in multiple
translation units. Sema is responsible for deciding when this is the
case, because it's already responsible for choosing the mangling
number.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151029 91177308-0d34-0410-b5e6-96231b3b80d8