[Sema] Fix a bug where clang doesn't detect uses of unavailable decls in C++ base or member initializers

[arphaman]

Thanks, LGTM

clang/lib/Sema/SemaAvailability.cpp

void Sema::DiagnoseUnguardedAvailabilityViolations(Decl *D) {
 
   if (auto *FD = D->getAsFunction()) {
     // FIXME: We only examine the pattern decl for availability violations now,
     // but we should also examine instantiated templates.
     if (FD->isTemplateInstantiation())
       return;
 
     Body = FD->getBody();
+
+    if (auto *CD = dyn_cast<CXXConstructorDecl>(FD))
+      for (const CXXCtorInitializer *CI : CD->inits())
+        DiagnoseUnguardedAvailability(*this, D).IssueDiagnostics(CI->getInit());
+
   } else if (auto *MD = dyn_cast<ObjCMethodDecl>(D))
     Body = MD->getBody();
   else if (auto *BD = dyn_cast<BlockDecl>(D))
     Body = BD->getBody();
 
   assert(Body && "Need a body here!");
 
   DiagnoseUnguardedAvailability(*this, D).IssueDiagnostics(Body);

clang/test/SemaCXX/attr-availability.cpp

+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.9.0 -std=c++11 -fsyntax-only -verify %s
+
+__attribute__((availability(macos, introduced = 10.0))) int init10();
+__attribute__((availability(macos, introduced = 11.0))) int init11(); // expected-note 2 {{'init11' has been marked as being introduced in macOS 11.0}}
+
+struct B0 {
+  B0(int);
+};
+
+struct B1 {
+  B1(int);
+};
+
+struct S : B0, B1 {
+  S() : B0(init10()),
+        B1(init11()), // expected-warning {{'init11' is only available on macOS 11.0}} expected-note {{enclose 'init11'}}
+        i0(init10()),
+        i1(init11())  // expected-warning {{'init11' is only available on macOS 11.0}} expected-note {{enclose 'init11'}}
+  {}
+  int i0, i1;
+};