Reimplement mlir::Identifier to be a wrapper around 'StringMapEntry*' instead...

Reimplement mlir::Identifier to be a wrapper around 'StringMapEntry*' instead of a wrapper around a 'const char*'.  This makes it so strref() can be computed without calling strlen, which is more efficient and less error-prone.  While here...

Summary:
..., reimplement DenseMapInfo<mlir::Identifier>::getHashValue in terms of mlir::hash_value(Identifier).

Both of these improvements were suggested by River, thanks!

Reviewers: rriddle!

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77999