[clang] Unify -mspeculative-load-hardening as a benign compiler option (#166640)

Before this patch, compilations using modules &
`-mspeculative-load-hardening` failed because it was not consistent
whether this option impacts module compatibility.

This repairs it by always treating it as benign. This was determined by
checking if the option enables any kind of preprocessor checks and
comparing how the compiler handles a similar option that impacts codegen
(-fvisibility=hidden) but is more widely used.

resolves: rdar://163985667

Author: cyndyishida

clang/include/clang/Basic/LangOptions.def

@@ -454,7 +454,7 @@ LANGOPT(BranchTargetEnforcement, 1, 0, NotCompatible, "Branch-target enforcement
 LANGOPT(BranchProtectionPAuthLR, 1, 0, NotCompatible, "Use PC as a diversifier using PAuthLR NOP instructions.")
 LANGOPT(GuardedControlStack, 1, 0, NotCompatible, "Guarded control stack enabled")
 
-LANGOPT(SpeculativeLoadHardening, 1, 0, NotCompatible, "Speculative load hardening enabled")
+LANGOPT(SpeculativeLoadHardening, 1, 0, Benign, "Speculative load hardening enabled")
 
 LANGOPT(RelativeCXXABIVTables, 1, 0, NotCompatible,
         "Use an ABI-incompatible v-table layout that uses relative references")

clang/test/ClangScanDeps/strip-codegen-args.m

@@ -16,6 +16,7 @@
 // CHECK-NOT:          "-flto"
 // CHECK-NOT:          "-fno-autolink"
 // CHECK-NOT:          "-mrelax-relocations=no"
+// CHECK-NOT:          "-mspeculative-load-hardening"
 // CHECK:            ]
 // CHECK:            "name": "A"
 // CHECK:          }
@@ -39,6 +40,11 @@
     "command": "clang -Imodules/A -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-modules -O2 -flto=full -fsyntax-only DIR/t3.m",
     "file": "DIR/t2.m"
   }
+  {
+    "directory": "DIR",
+    "command": "clang -Imodules/A -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-modules -O2 -mspeculative-load-hardening -fsyntax-only DIR/t3.m",
+    "file": "DIR/t3.m"
+  }
 ]
 
 //--- modules/A/module.modulemap