Workaround GCC bug PR78489 - SFINAE order is not respected.

This patch works around variant test failures which are new to
GCC 8. GCC 8 either doesn't perform SFINAE in lexical order, or
it doesn't halt after encountering the first failure. This
causes hard error to occur instead of substitution failure.

See gcc.gnu.org/PR78489

llvm-svn: 328261