Generate invariant_start/end on passed by-value const arguments.

[lvoufo]

Summary

Next incremental step from D13031.

lib/CodeGen/CGDecl.cpp

if (CGDebugInfo *DI = getDebugInfo()) {
     if (CGM.getCodeGenOpts().getDebugInfo()
           >= CodeGenOptions::LimitedDebugInfo) {
       DI->EmitDeclareOfArgVariable(&D, DeclPtr.getPointer(), ArgNo, Builder);
     }
   }
 
   if (D.hasAttr<AnnotateAttr>())
     EmitVarAnnotations(&D, DeclPtr.getPointer());
+
+  MarkWriteOnceWritten MWO(*this, D,
+                           DeclPtr.isValid() ? DeclPtr.getPointer() : nullptr);
 }

test/CodeGenCXX/const-invariant.cpp

#endif
   // CHECK-L-CO: call {{.*}} i8* @_Znwm(
   // CHECK-L-CO: call {{.*}}@llvm.invariant.start(i64 {{[0-9]+}}, i8*
   bar(*i_pc);
   foo(i_pc);           // May change *i_p.
   bar(*i_pc);
   // CHECK-L-CO: call {{.*}}@llvm.invariant.end({{.*}}, i64 {{[0-9]+}}, i8*
 }
 
+#ifdef LOCAL
+void ex1_parms(Const Type i) {
+// CHECK: @_Z9ex1_parms1A(
+  // CHECK-L-CO: store {{.*}} %i
+  // CHECK-L-CO: call {{.*}}@llvm.invariant.start(
+  bar(i);
+  foo(&i);           // May change i.
+  bar(i);
+  // CHECK-L-CO: call {{.*}}@llvm.invariant.end(
+}
+#endif
+
 // CHECK-G-CO-OBJ: call {{.*}}@llvm.invariant.start(i64 {{[0-9]+}}, i8* bitcast ({{.*}} @_ZL1j to i8*))
 // CHECK-G-CO-INT: store {{.*}}, {{.*}}* @_ZL1j