[AMDGPU] Null checking on TS to avoid crashing in clang tests.

- `test/Misc/backend-resource-limit-diagnostics.cl` crashes as null
  streamer is used.

llvm-svn: 364318

llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp

@@ -326,7 +326,8 @@ void AMDGPUAsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
 
     EmitVisibility(GVSym, GV->getVisibility(), !GV->isDeclaration());
     EmitLinkage(GV, GVSym);
-    getTargetStreamer()->emitAMDGPULDS(GVSym, Size, Align);
+    if (auto TS = getTargetStreamer())
+      TS->emitAMDGPULDS(GVSym, Size, Align);
     return;
   }