Driver: Don't warn on -mbranch-protection when linking

The -mbranch-protection definition in Options.td was not given a Group,
so this was causing clang to emit a -Wunused-command-line-argument
warning when this flag was passed to the linker driver.  This was a
problem, because some build systems, like cmake, automatically pass the
C flags to the linker.  Therefore, any program that was compiled with
-Werror and -mbranch-protection would fail to link with the error:

argument unused during compilation: '-mbranch-protection=standard' [-Werror,-Wunused-command-line-argument]

Reviewed By: vhscampos

Differential Revision: https://reviews.llvm.org/D121983