SamplePGO - Fix PR 25482 - Do not rely on llvm.dbg.cu for discriminators

The discriminators pass relied on the presence of llvm.dbg.cu to decide
whether to add discriminators, but this fails in the case where debug
info is only enabled partially when -fprofile-sample-use is active.

The reason llvm.dbg.cu is not present in these cases is to prevent
codegen from emitting debug info (as it is only used for the sample
profile pass).

This changes the discriminators pass to also emit discriminators even
when debug info is not being emitted.

llvm-svn: 252763