[CodeGen] Fix hashing for MO_ExternalSymbol MachineOperands.
We were hashing the string pointer, not the string, so two instructions
could be identical (isIdenticalTo), but have different hash codes.
This showed up as a very rare, non-deterministic assertion failure
rehashing a DenseMap constructed by MachineOutliner. So there's no
"real" testcase, just a unittest which checks that the hash function
I'm a little scared fixing this is going to cause a regression in
outlining or MachineCSE, but hopefully we won't run into any issues.
Differential Revision: https://reviews.llvm.org/D61975