[GlobalISel] Simplify StringRef parameters. NFC.
[GlobalISel] Simplify StringRef parameters. NFC.

'const' on StringRef parameters adds no guarantees. Remove it.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@294555 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit b2f3da065cbc679e5f281e2ef4bf5e38978c9471)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm