This patch is split from http://reviews.llvm.org/D15829.
It changes cc1 option -fprofile-generate to an enum option -fprofile-instrument=,
and it takes Clang as the default (and only, or now) value.
It also changes cc1 options -fprofile-generate= to -fprofile-instrument-path=.
The driver level option -fprofile-generate and -fprofile-generate= are intact.
This change will pave the way to integrate new PGO instrumentation in IR level.