Use cast<> instead of unchecked dyn_cast

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209310 91177308-0d34-0410-b5e6-96231b3b80d8
DebugInfo: Simplify subprogram declaration creation/references and accidentally refix PR11300.

Also simplifies the linkage name handling a little too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209311 91177308-0d34-0410-b5e6-96231b3b80d8