Debug Info: don't bother reinserting elements into maps we just retrieved them from

(no intended behavior change)

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@182403 91177308-0d34-0410-b5e6-96231b3b80d8
Debug Info: Simplify the code changed in r182403 to reduce indent & avoid a duplicate lookup in the insertion case

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@182404 91177308-0d34-0410-b5e6-96231b3b80d8