IRGen: use the llvm::TrailingObjects base class explicitly

Visual Studio is unable to follow the inheritance to find the method and objects
with:

  lib\IRGen\GenExistential.cpp(153): error C2903: 'totalSizeToAlloc': symbol is neither a class template nor a function template
  lib\IRGen\GenExistential.cpp(153): note: This diagnostic occurred in the compiler generated function 'const Derived *`anonymous-namespace'::ExistentialTypeInfoBase<Derived,Base>::create(llvm::ArrayRef<T>,As &&...)'
        with
        [
            T=const swift::ProtocolDecl *
        ]
  lib\IRGen\GenExistential.cpp(202): note: see reference to class template instantiation '`anonymous-namespace'::ExistentialTypeInfoBase<Derived,Base>' being compiled
  lib\IRGen\GenExistential.cpp(154): error C2760: syntax error: unexpected token 'const', expected 'expression'
  lib\IRGen\GenExistential.cpp(154): note: This diagnostic occurred in the compiler generated function 'const Derived *`anonymous-namespace'::ExistentialTypeInfoBase<Derived,Base>::create(llvm::ArrayRef<T>,As &&...)'
        with
        [
            T=const swift::ProtocolDecl *
        ]

Author: compnerd

lib/IRGen/GenExistential.cpp

@@ -150,8 +150,9 @@ namespace {
     create(ArrayRef<const ProtocolDecl *> protocols, As &&...args)
     {
       void *buffer = operator new(
-          ExistentialTypeInfoBase::template totalSizeToAlloc<
-            const ProtocolDecl *>(protocols.size()));
+          llvm::TrailingObjects<Derived, const ProtocolDecl *>::
+              template totalSizeToAlloc<const ProtocolDecl *>(
+                  protocols.size()));
       return new (buffer) Derived(protocols, std::forward<As>(args)...);
     }