Fix a warning building on my machine, NFC. warning: unused function 'compilePtxToCubinForTesting'

Summary: Also eliminate the redunant anon namespace around the already static function.

Reviewers: herhut

Subscribers: hiraditya, steven_wu, dexonsmith, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76627