[clang][SLH] Add __has_feature(speculative_load_hardening)

SLH doesn't support asm goto and is unlikely to ever support it. Users of asm
goto need a way to choose whether to use asm goto or fallback to an SLH
compatible code path when SLH is enabled. This feature flag will give users
this ability.

Tested via unit test

Reviewed By: mattdr

Differential Revision: https://reviews.llvm.org/D79733

Author: zbrid

clang/include/clang/Basic/Features.def

@@ -36,6 +36,7 @@
 #define EXTENSION(Name, Predicate)
 #endif
 
+FEATURE(speculative_load_hardening, LangOpts.SpeculativeLoadHardening)
 FEATURE(address_sanitizer,
         LangOpts.Sanitize.hasOneOf(SanitizerKind::Address |
                                    SanitizerKind::KernelAddress))

clang/include/clang/Basic/LangOptions.def

@@ -368,6 +368,8 @@ ENUM_LANGOPT(SignReturnAddressKey, SignReturnAddressKeyKind, 1, SignReturnAddres
              "Key used for return address signing")
 LANGOPT(BranchTargetEnforcement, 1, 0, "Branch-target enforcement enabled")
 
+LANGOPT(SpeculativeLoadHardening, 1, 0, "Speculative load hardening enabled")
+
 #undef LANGOPT
 #undef COMPATIBLE_LANGOPT
 #undef BENIGN_LANGOPT

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5361,8 +5361,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   Args.AddLastArg(CmdArgs, options::OPT_pthread);
 
-  if (Args.hasFlag(options::OPT_mspeculative_load_hardening, options::OPT_mno_speculative_load_hardening,
-                   false))
+  if (Args.hasFlag(options::OPT_mspeculative_load_hardening,
+                   options::OPT_mno_speculative_load_hardening, false))
     CmdArgs.push_back(Args.MakeArgString("-mspeculative-load-hardening"));
 
   RenderSSPOptions(TC, Args, CmdArgs, KernelOrKext);

clang/lib/Frontend/CompilerInvocation.cpp

@@ -3382,6 +3382,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   }
 
   Opts.BranchTargetEnforcement = Args.hasArg(OPT_mbranch_target_enforce);
+  Opts.SpeculativeLoadHardening = Args.hasArg(OPT_mspeculative_load_hardening);
 }
 
 static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {