[ARM,MVE] Fix user-namespace violation in arm_mve.h.

[simon_tatham]

Summary

We were generating the declarations of polymorphic intrinsics using
__attribute__((overloadable)). But overloadable is a valid
identifier for an end user to define as a macro in a C program, and if
they do that before including <arm_mve.h>, then we shouldn't cause a
compile error.

Fixed to spell the attribute name __overloadable__ instead.

[miyuki]

LGTM

clang/utils/TableGen/MveEmitter.cpp

for (bool Polymorphic : {false, true}) {
         // the type signatures that would have been legal. So all the
         // confusing machinery with __attribute__ is left out of the
         // error message, and the user sees something that's more or
         // less self-documenting: "here's a list of actually readable
         // type signatures for vfooq(), and here's why each one didn't
         // match your call".
 
         OS << "static __inline__ __attribute__(("
-           << (Polymorphic ? "overloadable, " : "")
+           << (Polymorphic ? "__overloadable__, " : "")
            << "__clang_arm_builtin_alias(__builtin_arm_mve_" << Int.fullName()
            << ")))\n"
            << RetTypeName << FunctionName << "(" << ArgTypesString << ");\n";
       }
     }
   }
   for (auto &part : parts)
     part << "\n";
for (bool Polymorphic : {false, true}) {
       for (const Type *ArgTypePtr : Int.argTypes())
         ArgTypeNames.push_back(ArgTypePtr->cName());
       std::string ArgTypesString =
           join(std::begin(ArgTypeNames), std::end(ArgTypeNames), ", ");
 
       // Emit the actual declaration. See MveEmitter::EmitHeader for detailed
       // comments
       OS << "static __inline__ __attribute__(("
-         << (Polymorphic ? "overloadable, " : "")
+         << (Polymorphic ? "__overloadable__, " : "")
          << "__clang_arm_builtin_alias(__builtin_arm_" << Int.builtinExtension()
          << "_" << Int.fullName() << ")))\n"
          << RetTypeName << FunctionName << "(" << ArgTypesString << ");\n";
     }
   }
 
   for (auto &part : parts)
     part << "\n";