Eliminate all uses of Identifier::is() in the source tree, this doesn't remove...

Eliminate all uses of Identifier::is() in the source tree, this doesn't remove the definition of it (yet).  NFC.

Reviewers: mravishankar, antiagainst, herhut, rriddle!

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78042