[ADT] Teach DenseMap to support StringRef keys.

While often you want to use something specialized like StringMap, when
the strings already have persistent storage a normal densemap over them
can be more efficient.

This can't go into StringRef.h because of really obnoxious header chains
from the hashing code to the endian detection code to CPU feature
detection code to StringMap.

llvm-svn: 240528