[clang-cl] Provide separate flags for all the /O variants

This provides better help text in "clang-cl /?".

Also it cleans things up a bit: previously "/Od" could be handled either
as a separate flag aliased to "-O0", or by the main optimization flag
processing in TranslateOptArg. With this patch, all the flags get
aliased back to /O so they're handled by TranslateOptArg.

Thanks to Nico for the idea!

Differential revision: https://reviews.llvm.org/D52266

llvm-svn: 342977