Fix use-of-invalid-memory found by Valgrind and Windows buildbots.

Fix use-of-invalid-memory found by Valgrind and Windows buildbots.
We were inserting a value into a std::vector<> while iterating over
it, which could cause the underlying memory to get deallocated
and reallocated.  While not the best solution, use an llvm::ImmutableList
for now as it is safely supports insertions during iteration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99058 91177308-0d34-0410-b5e6-96231b3b80d8