Fix crash while parsing variable template with variadic template arguments

Fix crash while parsing variable template with variadic template arguments

It is only a crash if the compiler optimize for this!=nullptr because
LocalInstantiationScope::getPartiallySubstitutedPack checks if 'this' is null
(This is crashing when clang is compiled with GCC6)

Differential Revision: http://reviews.llvm.org/D20511

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270845 91177308-0d34-0410-b5e6-96231b3b80d8