[X86] Don't allow '+f' as an inline asm constraint. (llvm#113871)

f cannot be used as an output constraint. We already errored for '=f'
but not '+f'.

Fixes llvm#113692.

Author: topperc

clang/lib/Basic/Targets/X86.cpp

@@ -1465,7 +1465,7 @@ bool X86TargetInfo::validateAsmConstraint(
     }
   case 'f': // Any x87 floating point stack register.
     // Constraint 'f' cannot be used for output operands.
-    if (Info.ConstraintStr[0] == '=')
+    if (Info.ConstraintStr[0] == '=' || Info.ConstraintStr[0] == '+')
       return false;
     Info.setAllowsRegister();
     return true;

clang/test/Sema/asm.c

@@ -204,6 +204,12 @@ double f_output_constraint(void) {
   return result;
 }
 
+double f_output_constraint_2(void) {
+  double result;
+  __asm("foo1": "+f" (result)); // expected-error {{invalid output constraint '+f' in asm}}
+  return result;
+}
+
 void fn1(void) {
   int l;
   __asm__(""