[IRGen] Check as early as possible for Clang decls we've seen before.

Previously, we were only doing this after the fast-path code that
handles decls without any executable code. This meant, however, that we
were potentially processing these decls multiple times. This is
definitely inefficient; it may even be a correctness issue, depending on
what amount of checking `HandleTopLevelDecl` does to see if it has
processed a particular decl before (which I'm not sure about either
way).

Author: martinboehme

lib/IRGen/GenClangDecl.cpp

@@ -37,15 +37,17 @@ class ClangDeclRefFinder
 } // end anonymous namespace
 
 void IRGenModule::emitClangDecl(const clang::Decl *decl) {
+  // Ignore this decl if we've seen it before.
+  if (!GlobalClangDecls.insert(decl->getCanonicalDecl()).second)
+    return;
+
   auto valueDecl = dyn_cast<clang::ValueDecl>(decl);
   if (!valueDecl || valueDecl->isExternallyVisible()) {
     ClangCodeGen->HandleTopLevelDecl(
                           clang::DeclGroupRef(const_cast<clang::Decl*>(decl)));
     return;
   }
 
-  if (!GlobalClangDecls.insert(decl->getCanonicalDecl()).second)
-    return;
   SmallVector<const clang::Decl *, 8> stack;
   stack.push_back(decl);