[driver] Add mips_Features_Group to Options to improve documentation sorting

Move all of the MIPS-only options into a new m_mips_Features_Group.
Nearly all other targets have most target-specific options grouped,
but MIPS does not.

The primary benefits are that the options will be listed together (and
thus identifiable as MIPS-specific even if they have no help string) in
the ClangCommandLineReference, and that Options.td is a bit more organized.

A secondary benefit is that a custom version of clang can more easily
hide/disable groups of options for unsupported targets.

Patch by Vince Del Vecchio

Differential Revision: https://reviews.llvm.org/D46450

llvm-svn: 331856