Complete list of Clang flags
I don't know if this is exactly what you want. Maybe more options are described elsewhere, but I think you are interested in the Clang frontend options. By default, the options displayed seem to describe the "GCC-compatible driver".
clang -cc1 --help should give you what you want.
For Clang, they are listed in the diagnostics reference, which can be found on the documentation website here
There are many hidden options in LLVM:
clang --help-hidden
opt --help-hidden