RFR: 8240259: Disable -Wshift-negative-value warnings

[Kim Barrett]

> On May 22, 2020, at 3:38 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
> 
> Looks good to me.

Thanks.

> 
> /Magnus
> 
> On 2020-05-22 03:54, Kim Barrett wrote:
>> Please review this change which disables warnings for left shift of a
>> negative value when compiling HotSpot with gcc or clang.  This warning
>> isn't helpful, given that all compilers seem to do the obvious thing,
>> and that obvious thing will soon-ish (C++20) be the specified thing.
>> See the CR for more details.
>> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8240259
>> 
>> Webrev:
>> https://cr.openjdk.java.net/~kbarrett/8240259/open.00/
>> 
>> Testing:
>> mach5 tier1.  Also tier1-5 with C++14 enabled.
>> 
>> Note that there are already occurrences of left shift of negative
>> values in our code, with no apparent consequences, even when testing
>> with C++11/14 enabled.