[hexagon] add a -mcabac flag

[bcain]

Summary

For v73 and later, clang users who wish to use the cabac instructions need
a way to add the 'cabac' target feature.

[bcain]

It seems extremely unlikely that this change to target-dependent flags caused this libc++ test failure w/an error referring to partial specialization.

@ldionne is it possible that special_mem_concepts::nothrow_sentinel_for.compile.pass.cpp is failing on the baseline?

In module 'std' imported from /home/libcxx-builder/.buildkite-agent/builds/7288bc8bb028-1/llvm-project/libcxx-ci/libcxx/test/libcxx/algorithms/specialized.algorithms/special.mem.concepts/nothrow_sentinel_for.compile.pass.cpp:14:
/home/libcxx-builder/.buildkite-agent/builds/7288bc8bb028-1/llvm-project/libcxx-ci/build/generic-modules/include/c++/v1/__concepts/same_as.h:29:34: error: partial specialization of 'common_reference<_Tp, _Up>' must be imported from module 'std.type_traits.common_reference' before it is required
concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
                                 ^
/home/libcxx-builder/.buildkite-agent/builds/7288bc8bb028-1/llvm-project/libcxx-ci/libcxx/test/libcxx/algorithms/specialized.algorithms/special.mem.concepts/nothrow_sentinel_for.compile.pass.cpp:23:80: note: while substituting into concept arguments here; substitution failures not allowed in concept arguments
constexpr bool ntsf_subsumes_sf(std::ranges::__nothrow_sentinel_for<char*> auto) requires true {
                                                                               ^
/home/libcxx-builder/.buildkite-agent/builds/7288bc8bb028-1/llvm-project/libcxx-ci/build/generic-modules/include/c++/v1/__type_traits/common_reference.h:133:40: note: partial specialization declared here is not reachable
template <class _Tp, class _Up> struct common_reference<_Tp, _Up> : __common_reference_sub_bullet1<_Tp, _Up> {};
                                       ^
1 error generated.

[kparzysz]

The cxx tests passed for me.

clang/include/clang/Driver/Options.td

 def mnvj : Flag<["-"], "mnvj">, Group<m_hexagon_Features_Group>,
   Flags<[CC1Option]>, HelpText<"Enable generation of new-value jumps">;
 def mno_nvj : Flag<["-"], "mno-nvj">, Group<m_hexagon_Features_Group>,
   Flags<[CC1Option]>, HelpText<"Disable generation of new-value jumps">;
 def mnvs : Flag<["-"], "mnvs">, Group<m_hexagon_Features_Group>,
   Flags<[CC1Option]>, HelpText<"Enable generation of new-value stores">;
 def mno_nvs : Flag<["-"], "mno-nvs">, Group<m_hexagon_Features_Group>,
   Flags<[CC1Option]>, HelpText<"Disable generation of new-value stores">;
+def mcabac: Flag<["-"], "mcabac">, Group<m_hexagon_Features_Group>,
+  HelpText<"Enable CABAC instructions">;
 
 // SPARC feature flags
 def mfpu : Flag<["-"], "mfpu">, Group<m_sparc_Features_Group>;
 def mno_fpu : Flag<["-"], "mno-fpu">, Group<m_sparc_Features_Group>;
 def mfsmuld : Flag<["-"], "mfsmuld">, Group<m_sparc_Features_Group>;
 def mno_fsmuld : Flag<["-"], "mno-fsmuld">, Group<m_sparc_Features_Group>;
 def mpopc : Flag<["-"], "mpopc">, Group<m_sparc_Features_Group>;
 def mno_popc : Flag<["-"], "mno-popc">, Group<m_sparc_Features_Group>;

clang/test/Driver/hexagon-toolchain-elf.c

 // RUN: %clang -### -target hexagon-unknown-elf -ffixed-r19 %s 2>&1 \
 // RUN:        | FileCheck --check-prefix=CHECK370 %s
 // CHECK370: "-target-feature" "+reserved-r19"
 // RUN: %clang -### -target hexagon-unknown-elf %s 2>&1 \
 // RUN:        | FileCheck --check-prefix=CHECK371 %s
 // CHECK371-NOT: "+reserved-r19"
 
 // -----------------------------------------------------------------------------
+// mcabac
+// -----------------------------------------------------------------------------
+// RUN: %clang -### -target hexagon-unknown-elf -mcabac %s 2>&1 \
+// RUN:        | FileCheck --check-prefix=CHECK372 %s
+// CHECK372: "-target-feature" "+cabac"
+// RUN: %clang -### -target hexagon-unknown-elf %s 2>&1 \
+// RUN:        | FileCheck --check-prefix=CHECK373 %s
+// CHECK373-NOT: "+cabac"
+
+// -----------------------------------------------------------------------------
 // Misc Defaults
 // -----------------------------------------------------------------------------
 // RUN: %clang -### -target hexagon-unknown-elf \
 // RUN:   -ccc-install-dir %S/Inputs/hexagon_tree/Tools/bin \
 // RUN:   -mcpu=hexagonv60 \
 // RUN:   %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHECK380 %s
 // CHECK380:      "-cc1"