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

[tstellar]

Summary

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]

[vhscampos]

LGTM, thanks

clang/include/clang/Driver/Options.td

def mvscale_max_EQ : Joined<["-"], "mvscale-max=">,
   HelpText<"Specify the vscale maximum. Defaults to the"
            " vector length agnostic value of \"0\". (AArch64 only)">,
   MarshallingInfoInt<LangOpts<"VScaleMax">>;
 
 def msign_return_address_EQ : Joined<["-"], "msign-return-address=">,
   Flags<[CC1Option]>, Group<m_Group>, Values<"none,all,non-leaf">,
   HelpText<"Select return address signing scope">;
 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=">,
   HelpText<"Select straight-line speculation hardening scope">;
 
 def msimd128 : Flag<["-"], "msimd128">, Group<m_wasm_Features_Group>;
 def mno_simd128 : Flag<["-"], "mno-simd128">, Group<m_wasm_Features_Group>;
 def mrelaxed_simd : Flag<["-"], "mrelaxed-simd">, Group<m_wasm_Features_Group>;

clang/test/Driver/Inputs/main.c

+int main(int argc, char **argv) {
+  return 0;
+}

clang/test/Driver/aarch64-security-options.c

 // RUN: FileCheck %s --check-prefix=CONFLICT --check-prefix=WARN
 
 // RUN: %clang -target aarch64--none-eabi -c %s -### -msign-return-address=foo     2>&1 | \
 // RUN: FileCheck %s --check-prefix=BAD-RA-PROTECTION --check-prefix=WARN
 
 // RUN: %clang -target aarch64--none-eabi -c %s -### -mbranch-protection=bar     2>&1 | \
 // RUN: FileCheck %s --check-prefix=BAD-BP-PROTECTION --check-prefix=WARN
 
+// RUN: %clang -target aarch64--none-eabi -o %t-main.o -mbranch-protection=standard -c %S/Inputs/main.c
+// RUN: %clang -target aarch64--none-eabi -o %t-main -mbranch-protection=standard %t-main.o 2>&1 | \
+// RUN: FileCheck --allow-empty %s --check-prefix=LINKER-DRIVER
+
 // WARN-NOT: warning: ignoring '-mbranch-protection=' option because the 'aarch64' architecture does not support it [-Wbranch-protection]
 
 // RA-OFF: "-msign-return-address=none"
 // RA-NON-LEAF: "-msign-return-address=non-leaf"
 // RA-ALL: "-msign-return-address=all"
 
 // KEY-A: "-msign-return-address-key=a_key"
 // KEY-NOT: "-msign-return-address-key"
 
 // BTE-OFF-NOT: "-mbranch-target-enforce"
 // BTE-ON: "-mbranch-target-enforce"
 
 // CONFLICT: "-msign-return-address=none"
 
 // BAD-RA-PROTECTION: invalid branch protection option 'foo' in '-msign-return-address={{.*}}'
 // BAD-BP-PROTECTION: invalid branch protection option 'bar' in '-mbranch-protection={{.*}}'
 
 // BAD-B-KEY-COMBINATION: invalid branch protection option 'b-key' in '-mbranch-protection={{.*}}'
 // BAD-LEAF-COMBINATION: invalid branch protection option 'leaf' in '-mbranch-protection={{.*}}'
+
+// Check that the linker driver doesn't warn about -mbranch-protection=standard
+// as an unused option.
+// LINKER-DRIVER-NOT: warning: