Revert "[Support] change StringMap hash function from djbHash to xxHash"
This reverts commit
d768b97424f9e1a0aae45440a18b99f21c4027ce.
Causes sanitizer failure: https://lab.llvm.org/buildbot/#/builders/238/builds/1114
```
/b/sanitizer-aarch64-linux-bootstrap-ubsan/build/llvm-project/llvm/lib/Support/xxhash.cpp:107:12: runtime error: applying non-zero offset 8 to null pointer
#0 0xaaaab28ec6c8 in llvm::xxHash64(llvm::StringRef) /b/sanitizer-aarch64-linux-bootstrap-ubsan/build/llvm-project/llvm/lib/Support/xxhash.cpp:107:12
#1 0xaaaab28cbd38 in llvm::StringMapImpl::LookupBucketFor(llvm::StringRef) /b/sanitizer-aarch64-linux-bootstrap-ubsan/build/llvm-project/llvm/lib/Support/StringMap.cpp:87:28
```
Probably causes test failure in `warn-unsafe-buffer-usage-fixits-local-var-span.cpp`: https://lab.llvm.org/buildbot/#/builders/60/builds/10619
Probably causes reverse-iteration test failure in `test-output-format.ll`: https://lab.llvm.org/buildbot/#/builders/54/builds/3545