Unbreak the build after r211244.

These operator overloads were unneeded (once the const correctness in
PooledStringPtr was corrected) and inefficient (since the strings are
pooled the pointers can just be compared for equality instead of doing
strcmp).

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@211245 91177308-0d34-0410-b5e6-96231b3b80d8