Clear the whole table including the tombstones, since the tombstone count will

Clear the whole table including the tombstones, since the tombstone count will
be set to zero that is what it was intended. Should improve performance of
the data structure when clear is invoked frequently (both compile time and
memory usage).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175799 91177308-0d34-0410-b5e6-96231b3b80d8