[clang] accept inline asm 'p' constraint on outputs

[nickdesaulniers]

Summary

GCC allows this; we reject it.

Link: https://reviews.llvm.org/D145416#inline-1404524

[nickdesaulniers]

• rebase, additional unit test

clang/lib/Basic/TargetInfo.cpp

while (*Name) {
     case 'm': // memory operand.
     case 'o': // offsetable memory operand.
     case 'V': // non-offsetable memory operand.
     case '<': // autodecrement memory operand.
     case '>': // autoincrement memory operand.
       Info.setAllowsMemory();
       break;
     case 'g': // general register, memory operand or immediate integer.
+    case 'p': // address operand.
     case 'X': // any operand.
       Info.setAllowsRegister();
       Info.setAllowsMemory();
       break;
     case ',': // multiple alternative constraint.  Pass it.
       // Handle additional optional '=' or '+' modifiers.
       if (Name[1] == '=' || Name[1] == '+')
         Name++;

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}}
+  asm ("":"=p"(t), "=p"(p));
+  asm ("":"=p"(&t)); // expected-error{{invalid lvalue in asm output}}
 }