Bug#949024: g++: internal compiler error on beignet

["Rebecca N. Palmer" <rebecca_palmer@zoho.com>]

Control: retitle -1 g++: ICE at cp/pt.c:15779 on beignet in -std=gnu++14 
or higher

This also happens with LLVM 9 (which makes sense given that the error 
message points to code in beignet itself):

                  LLVM 9        LLVM 10
-std=c++0x       success    std::index_sequence fail
-std=c++11       success    std::index_sequence fail
none (gnu++14)      ICE         ICE
-std=c++17      not tested      ICE
none, gcc-snapshot  ICE       not tested

("std::index_sequence fail" above means
/usr/lib/llvm-10/include/llvm/ADT/STLExtras.h:559:49: error: 
'std::index_sequence' has not been declared
I suspect this is "LLVM 10 needs at least c++14" and not a bug, but as 
it happens first, I don't know whether those combinations would have the 
ICE.)

It probably *isn't* the upstream GCC bug I mentioned above, as the line 
number doesn't match: in Debian gcc 9, upstream bugs 91467 and 92583 are 
both cp/pt.c:15538, 92625 is cp/pt.c:16142, and 92948 is cp/pt.c:15774.

gcc-snapshot also has this bug (at cp/pt.c:16332).