Switch more of Sema::CheckInitializerTypes over to

Switch more of Sema::CheckInitializerTypes over to
InitializationSequence. Specially, switch initialization of a C++
class type (either copy- or direct-initialization).

Also, make sure that we create an elidable copy-construction when
performing copy initialization of a C++ class variable. Fixes PR5826.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91750 91177308-0d34-0410-b5e6-96231b3b80d8