add test case

[nickdesaulniers]

• rebase, additional unit test

clang/test/Sema/inline-asm-validate.c

 // RUN: %clang_cc1 -triple arm64-apple-macosx10.8.0 -fsyntax-only -verify %s
 
 unsigned t, r, *p;
 
 void foo (void) {
   __asm__ __volatile__( "stxr   %w[_t], %[_r], [%[_p]]" : [_t] "=&r" (t) : [_p] "p" (p), [_r] "r" (r) : "memory"); // expected-warning{{value size does not match register size specified by the constraint and modifier}} expected-note {{use constraint modifier "w"}}
   // Ensure we accept pointer and non-pointer arguments to "p" constrained
   // inputs.
   asm (""::"p"(t), "p"(p), "p"(&t));
+  // TODO: Ensure we accept p output constraints.
+  asm ("":"=p"(t)); // expected-error{{invalid output constraint '=p' in asm}}
+  asm ("":"=p"(p)); // expected-error{{invalid output constraint '=p' in asm}}
+  asm ("":"=p"(&t)); // expected-error{{invalid output constraint '=p' in asm}}
 }