[PGO] Enable `-fprofile-update` for `-fprofile-generate`

Currently, the `-fprofile-udpate` is ignored when `-fprofile-generate` is in effect. This patch enables `-fprofile-update` for `-fprofile-generate`. This patch continues the work from https://reviews.llvm.org/D87737, which added `-fprofile-update` in the first place.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D157280