GCC: Make some operators of QueueSerial non constexpr Fix build breakage for GCC because of calling non constexpr function from a constexpr function. The error example: third_party/angle/src/libANGLE/renderer/serial_utils.h:214:40: error: call to non-'constexpr' function 'rx::Serial rx::AtomicQueueSerialFixedArray::operator[](rx::SerialIndex) const' Bug: chromium:819294 Change-Id: Ic09be54ce9418ab3f7d947eaa27c97b55c3c2bab Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4098527 Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Charlie Lao <cclao@google.com>