[SampleFDO] Rename llvm-profdata flag -partial-profile to -gen-partial-profile.

The internal flag -partial-profile in llvm conflicts with the flag with
the same name in llvm-profdata. The conflict happens in builds with
LLVM_LINK_LLVM_DYLIB enabled. In this case the tools are linked with libLLVM
and we end up with two definitions for the same cl::opt.

The patch renames llvm-profdata flag -partial-profile to -gen-partial-profile.