[docs][HIP] Document offload PGO workflow (#200208)
Add a section to HIPSupport.rst describing IR-level profile-guided
optimization for HIP device code. -fprofile-generate instruments both
host and device; the runtime writes a host .profraw and one set of
device .profraw files per --offload-arch= value, with the standard
LLVM_PROFILE_FILE substitutions applying to both. Host and each
per-architecture device profile are merged independently with
llvm-profdata, and the use-phase build feeds them back via -Xarch_host
-fprofile-use= and -Xarch_<gpu-arch> -fprofile-use= (with -Xarch_device
as a single-arch shorthand).
Also add a CUDA/HIP Language Changes entry in ReleaseNotes.rst.
