[ADT] Fix StringSet::insert() to not allocate on every lookup.
 - The previous implementation always constructed the StringMap entry, even if
   the key was present in the set.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177178 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed