r207997 - Fix pr19653.

[Rafael Espíndola]

> I was hoping we'd say which section the alias would end up in here -- the
> requested section should be obvious from the snippet.

Not necessarily. For example:

>> +void test3_bar() { }
>> +void test3_foo() __attribute__((section("test"))); // expected-warning
>> {{alias will not be in section 'test' but in the same section as the
>> aliasee}}
>> +void test3_foo() __attribute__((alias("test3_bar")));

In here the aliasee has no explicit section, so we will only know it
during llvm's codegen.

Cheers,
Rafael