add test case

[nickdesaulniers]

• update commit message

clang/test/CodeGen/inline-asm-p-constraint.c

 // RUN: %clang_cc1 -emit-llvm %s -o - -triple aarch64-linux-gnu | FileCheck %s
 void foo(void* ptr) {
   asm volatile("prfm pldl1keep, %a0\n" : : "p" (ptr));
   // CHECK:     call void asm sideeffect "prfm pldl1keep, ${0:a}\0A", "r"(ptr %0)
   // CHECK-NOT: call void asm sideeffect "prfm pldl1keep, ${0:a}\0A", "p"(ptr %0)
 }
+
+int *current_task;
+long x;
+void bar (void) {
+  asm ("":"=r"(x):"p"(&current_task));
+}
+// CHECK: %0 = call i64 asm "", "=r,r"(ptr @current_task) #[[ATTR:[0-9]+]]
+// FIXME: should be memory(read)
+// CHECK: attributes #[[ATTR]] = { nounwind memory(none) }