[Driver] Place -mharden-sls= in m_Group

Fix #63237

With only a link action, we claim all CompileOnly_Group options (including -f*,
-m*, -i*, etc). -mharden-sls= is not in a Group and therefore not claimed,
leading to a spurious -Wunused-command-line-argument warning, and after
5548843, an error.

clang/include/clang/Driver/Options.td

@@ -3846,7 +3846,7 @@ def mbranch_protection_EQ : Joined<["-"], "mbranch-protection=">,
   Group<m_Group>,
   HelpText<"Enforce targets of indirect branches and function returns">;
 
-def mharden_sls_EQ : Joined<["-"], "mharden-sls=">,
+def mharden_sls_EQ : Joined<["-"], "mharden-sls=">, Group<m_Group>,
   HelpText<"Select straight-line speculation hardening scope (ARM/AArch64/X86"
            " only). <arg> must be: all, none, retbr(ARM/AArch64),"
            " blr(ARM/AArch64), comdat(ARM/AArch64), nocomdat(ARM/AArch64),"

clang/test/Driver/x86-target-features.c

@@ -364,3 +364,6 @@
 // SLS-IJMP-DAG: "-target-feature" "+harden-sls-ijmp"
 // NO-SLS-NOT: "+harden-sls-
 // BAD-SLS: unsupported argument '{{[^']+}}' to option '-mharden-sls='
+
+// RUN: touch %t.o
+// RUN: %clang -fdriver-only -Werror --target=x86_64-pc-linux-gnu -mharden-sls=all %t.o -o /dev/null 2>&1 | count 0