[backport:2.6] [!528] Work around a nasty compiler bug in GCC 6.3

GCC 6.3 errors out with an ICE or simply hangs when instantiating a certain variant of operator^= on BitSetVectorReference. This patch works around the problem by forcing the actual XOR instruction into a separate function for that compiler. The workaround is probably horribly slow, but it shouldn't have any impact on unaffected compilers.

This probably needs backporting.

See merge request !528 (merged)

(cherry picked from commit 9a9fceea)

528b5f36 Work around a nasty compiler bug in GCC 6.3

mentioned in commit 8f902169

merged