[ProfileData] Make a utility method public, NFC

The swift frontend needs to be able to look up PGO function name
variables based on the original raw function name. That's because it's
not possible to create PGO function name variables while emitting swift
IR. Instead, we have to create the name variables while lowering swift
IR to llvm IR, at which point we fix up all calls to the increment
intrinsic to point to the right name variable.

llvm-svn: 263662