[Sema] Fix a crash that occurs when a variable template is initialized

with a generic lambda.

This patch fixes Sema::InstantiateVariableInitializer to switch to the
context of the variable before instantiating its initializer, which is
necessary to set the correct type for VarTemplateSpecializationDecl.

This is the first part of the patch that was reviewed here:
http://reviews.llvm.org/D19175

rdar://problem/23440346

llvm-svn: 267956