[SYCL] Move deferred emit check to after "Can be emitted" checks.

The previous implementation caused function declarations to be attempted
to be emitted if they were required to be emitted (such as in the case
of -femit-all-decls), which caused a crash.

This patch moves the check until after the 'can be emitted' checks for
function/var decls, and limits it to only functions that are
sycl-devices (or, in the case of implicit attributes, called from a
sycl-device function).

Author: Erich Keane

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2642,11 +2642,6 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     }
   }
 
-  if (LangOpts.SYCLIsDevice && MustBeEmitted(Global)) {
-    addDeferredDeclToEmit(GD);
-    return;
-  }
-
   // Ignore declarations, they will be emitted on their first use.
   if (const auto *FD = dyn_cast<FunctionDecl>(Global)) {
     // Forward declarations are emitted lazily on first use.
@@ -2698,6 +2693,13 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     }
   }
 
+  // clang::ParseAST ensures that we emit the SYCL devices at the end, so
+  // anything that is a device (or indirectly called will be handled later.
+  if (LangOpts.SYCLIsDevice && Global->hasAttr<SYCLDeviceAttr>()){
+    addDeferredDeclToEmit(GD);
+    return;
+  }
+
   // Defer code generation to first use when possible, e.g. if this is an inline
   // function. If the global must always be emitted, do it eagerly if possible
   // to benefit from cache locality.

clang/test/CodeGenSYCL/emit-all-decls-crash.cpp

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsycl -fsycl-is-device -triple spir64-unknown-unknown-sycldevice -disable-llvm-passes -emit-llvm %s -femit-all-decls -o - | FileCheck %s
+
+// This should not crash and we should not emit this declaration, even though
+// we have 'emit-all-decls'.
+// CHECK-NOT: define
+void foo(void);